Magnesium-sulfur secondary battery containing a metal polysulfide-preloaded active cathode layer

ABSTRACT

A rechargeable magnesium-sulfur cell comprising an anode layer, an electrolyte, a metal polysulfide-preloaded active cathode layer, wherein the active cathode layer comprises: (a) an integral porous structure having massive surfaces (specific surface area &gt;100 m 2 /g) or pores with a size from 1.0 nm to 100 nm and wherein multiple particles; and (b) a metal polysulfide, M x S y , preloaded in the pores or deposited on the massive surfaces, wherein x is an integer from 1 to 3 and y is an integer from 1 to 10, and M is a metal element selected from an alkali metal, an alkaline metal, a transition metal, a metal from groups 13 to 17 of the periodic table, or a combination thereof. The metal polysulfide is in a form of solid-state thin coating or small particles with a thickness or diameter less than 50 nm.

FIELD OF THE INVENTION

The present invention is directed at a rechargeable magnesium-sulfurcell, also referred to as the Mg—S secondary cell.

BACKGROUND

With the rapid development of hybrid (HEY), plug-in hybrid electricvehicles (HEV), and all-battery electric vehicles (EV), there is anurgent need for a rechargeable battery having a high specific energy,high energy density, high rate capability, long cycle life, and safety.One of the most promising energy storage devices is the lithium-sulfur(Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and thatof S is 1,675 mAh/g. In its simplest form, a Li—S cell consists ofelemental sulfur as the positive electrode and lithium as the negativeelectrode active materials. The lithium-sulfur cell operates with aredox couple, described by the reaction S₈+16Li

8Li₂S that lies near 2.2 V with respect to Li⁺/Li⁰. This electrochemicalpotential is approximately ⅔ of that exhibited by conventional positiveelectrodes (e.g. LiMnO₄) of a lithium-ion battery. However, thisshortcoming is offset by the very high theoretical capacities of both Liand S. Thus, compared with conventional intercalation-based Li-ionbatteries, Li—S cells have the opportunity to provide a significantlyhigher energy density (a product of capacity and voltage). Assumingcomplete reaction to Li₂S, energy densities values can approach 2,500Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and Sweights or volumes. If based on the total cell weight or volume, theenergy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l,respectively. However, the current Li-sulfur cells reported by industryleaders in sulfur cathode technology have a maximum cell specific energyof 250-400 Wh/kg (based on the total cell weight), which is far belowwhat is possible.

Despite its considerable advantages, the Li—S cell is plagued withseveral major technical problems that have thus far hindered itswidespread commercialization:

-   (1) Lithium metal cells still have dendrite formation and related    internal shorting issues.-   (2) Sulfur or sulfur-containing organic compounds are highly    insulating, both electrically and ionically. To enable a reversible    electrochemical reaction at high current densities or    charge/discharge rates, the sulfur must maintain intimate contact    with an electrically conductive additive. Various carbon-sulfur    composites have been utilized for this purpose, but only with    limited success owing to the limited scale of the contact area.    Typical reported capacities are between 300 and 550 mAh/g (based on    the cathode carbon-sulfur composite weight) at moderate rates.-   (3) The cell tends to exhibit significant capacity decay during    discharge-charge cycling. This is mainly due to the high solubility    of the lithium polysulfide anions formed as reaction intermediates    during both discharge and charge processes in the polar organic    solvents used in electrolytes. During cycling, the lithium    polysulfide anions can migrate through the separator to the Li    negative electrode whereupon they are reduced to solid precipitates    (Li₂S₂ and/or Li₂S), causing active mass loss. In addition, the    solid product that precipitates on the surface of the positive    electrode during discharge becomes electrochemically irreversible,    which also contributes to active mass loss.-   (4) More generally speaking, a significant drawback with cells    containing cathodes comprising elemental sulfur, organosulfur and    carbon-sulfur materials relates to the dissolution and excessive    out-diffusion of soluble sulfides, polysulfides, organo-sulfides,    carbon-sulfides and/or carbon-polysulfides (hereinafter referred to    as anionic reduction products) from the cathode into the rest of the    cell. This phenomenon is commonly referred to as the Shuttle Effect.    This process leads to several problems: high self-discharge rates,    loss of cathode capacity, corrosion of current collectors and    electrical leads leading to loss of electrical contact with active    cell components, fouling of the anode surface, and clogging of the    pores in the cell membrane separator which leads to loss of ion    transport and large increases in internal resistance in the cell.

In response to these challenges, new electrolytes, protective films forthe lithium anode, and solid electrolytes have been developed. Someinteresting cathode developments have been reported recently to containlithium polysulfides; but, their performance still fall short of what isrequired for practical applications.

For instance, Ji, et al reported that cathodes based on nanostructuredsulfur/meso-porous carbon materials could overcome these challenges to alarge degree, and exhibit stable, high, reversible capacities with goodrate properties and cycling efficiency [Xiulei Ji, Kyu Tae Lee, & LindaF. Nazar, “A highly ordered nanostructured carbon-sulphur cathode forlithium-sulphur batteries,” Nature Materials 8, 500-506 (2009)].However, the fabrication of the proposed highly ordered meso-porouscarbon structure requires a tedious and expensive template-assistedprocess. It is also challenging to load a large proportion of sulfurinto these meso-scaled pores using a physical vapor deposition orsolution precipitation process.

Zhang, et al. (US Pub. No. 2014/0234702; Aug. 21, 2014) makes use of achemical reaction method of depositing S particles on surfaces ofisolated graphene oxide (GO) sheets. But, this method is incapable ofcreating a large proportion of S particles on GO surfaces (i.e.typically <66% of S in the GO-S nanocomposite composition). Theresulting Li—S cells also exhibit poor rate capability; e.g. thespecific capacity of 1,100 mAh/g (based on S weight) at 0.02 C rate isreduced to <450 mAh/g at 1.0 C rate. It may be noted that the highestachievable specific capacity of 1,100 mAh/g represents a sulfurutilization efficiency of only 1,100/1,675=65.7% even at such a lowcharge/discharge rate (0.02 C means completing the charge or dischargeprocess in 1/0.02=50 hours; 1 C=1 hour, 2 C=½ hours, and 3 C=⅓ hours,etc.) Further, such a S-GO nanocomposite cathode-based Li—S cellexhibits very poor cycle life, with the capacity typically dropping toless than 60% of its original capacity in less than 40 charge/dischargecycles. Such a short cycle life makes this Li—S cell not useful for anypractical application. Another chemical reaction method of depositing Sparticles on graphene oxide surfaces is disclosed by Wang, et al. (USPub. No. 2013/0171339; Jul. 4, 2013). This Li—S cell still suffers fromthe same problems.

A solution precipitation method was disclosed by Liu, et al. (US Pub.No. 2012/0088154; Apr. 12/2012) to prepare graphene-sulfurnanocomposites (having sulfur particles adsorbed on GO surfaces) for useas the cathode material in a Li—S cell. The method entails mixing GOsheets and S in a solvent (CS₂) to form a suspension. The solvent isthen evaporated to yield a solid nanocomposite, which is then ground toyield nanocomposite powder having primary sulfur particles with anaverage diameter less than approximately 50 nm. Unfortunately, thismethod does not appear to be capable of producing S particles less than40 nm. The resulting Li—S cell exhibits very poor cycle life (a 50%decay in capacity after only 50 cycles). Even when these nanocompositeparticles are encapsulated in a polymer, the Li—S cell retains less than80% of its original capacity after 100 cycles. The cell also exhibits apoor rate capability (specific capacity of 1,050 mAh/g(S wt.) at 0.1 Crate, dropped to <580 mAh/g at 1.0 C rate). Again, this implies that alarge proportion of S did not contribute to the lithium storage,resulting in a low S utilization efficiency.

Furthermore, all of the aforementioned methods involve depositing Sparticles onto surfaces of isolated graphene sheets. The presence of Sparticles or coating (one of the most insulating materials) adhered tographene surfaces would make the resulting electrode structurenon-conducting when multiple S-bonded graphene sheets are packedtogether. These S particles prevent graphene sheets from contacting eachother, making it impossible for otherwise conducting graphene sheets toform a 3-D network of electron-conducting paths in the cathode. Thisunintended and unexpected outcome is another reason why these prior artLi—S cells have performed so poorly.

Despite the various approaches proposed for the fabrication of highenergy density Li—S cells, there remains a need for cathode materials,production processes, and cell operation methods that retard theout-diffusion of S or lithium polysulfide from the cathode compartmentsinto other components in these cells, improve the utilization ofelectro-active cathode materials (S utilization efficiency), and providerechargeable Li—S cells with high capacities over a large number ofcycles.

Hence, there has been strong and continued demand for batteries capableof storing more energy (Wh/l or Wh/kg) and delivering more power (W/kgor W/l) than current rechargeable Li-ion batteries and Li—S batteries.One possible route to meeting this demand is to utilize divalentmagnesium ion (Mg²⁺), rather than the monovalent lithium cation (Li⁺)because magnesium enables nearly twice as much charge to be transferred,per weight or volume, as Li⁺ thus enabling higher energy density.Further, magnesium metal and Mg-containing alloys or compounds are moreabundant and readily available, potentially enabling significant costreduction relative to Li metal batteries. Unfortunately, in general, thecathode active materials capable of storing Mg ions exhibit even lowerspecific capacity (typically <200 mAh/g and more typically <150 mAh/g)as compared to the current cathode active materials for lithium-ioncells. These cathode active materials proposed for use in a Mg-ion cellinclude: Chevrel phase Mo₆S₈, MnO₂, CuS, Cu₂S, Ag₂S, CrS₂, and VOPO₄;layered compounds TiS₂, V₂O₅, MgVO₃, MoS₂, MgV₂O₅, and MoO₃; Spinelstructured compounds CuCr₂S₄, MgCr₂S₄, MgMn₂O₄, and Mg₂MnO₄; NASICONstructured compounds MgFe₂(PO₄)₃ and MgV₂(PO₄)₃; Olivine structuredcompounds MgMnSiO₄ and MgFe₂(PO₄)₂; Tavorite structured compoundMg_(0.5)VPO₄F; pyrophosphates TiP₂O₇ and VP₂O₇; and FeF₃.

In comparison with the aforementioned compounds, sulfur remains a primecandidate cathode active material for a Mg metal secondary battery usingMg metal (>99.9% Mg) or lightly alloyed Mg metal (containing 70-99.9% Mgin the alloy) as the anode active material. In other words, Mg—S wouldbe an ideal high-energy cell. However, the Mg—S cell is not without aproblem. In fact, the Mg—S cell has even more problems than the Li—Scell does. There are some similar issues between Mg—S and Li—Sbatteries, such as: (i) low active material utilization rate, (ii) poorcycle life, and (iii) low Coulombic efficiency. Again, these drawbacksarise mainly from insulating nature of S, dissolution of S and metalpolysulfide intermediates in liquid electrolytes (and related Shuttleeffect), and large volume change during battery charging/discharging.

However, there are very serious issues that are unique to Mg—S batteriesthat Li—S batteries do not have: (a) The Li metal at the anode of theLi—S cell can form a passivating layer that is permeable to Li⁺ ions,but the passivating layer on the Mg metal at the anode of the Mg—S cellis not permeable to the Mg²⁺ ions, making the dissolution/re-plating ofMg²⁺ ions (discharge/re-charge of the Mg metal secondary battery)difficult or impossible in most of the known electrolytes; (b) Due tothis reason, there are only a limited number of electrolytes that arecompatible with Mg metal at the anode; and (c) Unfortunately, theseelectrolytes are nucleophilic, making them incompatible with theelectrophilic sulfur cathode, which is known to require anon-nucleophilic electrolyte.

Hence, a specific object of the present invention is to provide arechargeable magnesium-sulfur cell based on rational materials andbattery designs that overcome or significantly reduce the followingissues commonly associated with Mg—S cells: (a) impermeable passivatinglayer issue of Mg metal anode; (b) electrolyte incompatibility issue;(c) extremely low electric and ionic conductivities of sulfur, requiringlarge proportion (typically 30-55%) of non-active conductive fillers andhaving significant proportion of non-accessible or non-reachable sulfuror metal polysulfides); (d) dissolution of S and metal polysulfide inelectrolyte and migration of polysulfides from the cathode to the anode,resulting in active material loss and capacity decay (the shuttleeffect); and (e) short cycle life.

By addressing most of the aforementioned issues, the present inventionprovides a technically feasible rechargeable magnesium-sulfur batterythat exhibits an exceptionally high specific energy (Wh/kg) or energydensity (Wh/l) for a long cycle life. Thus, one particular technicalgoal of the present invention is to provide a magnesium-sulfur cell witha cell specific energy greater than 400 Wh/Kg, preferably greater than600 Wh/Kg, and more preferably greater than 800 Wh/Kg (all based on thetotal cell weight).

Another object of the present invention is to provide a Mg—S cell thatexhibits a high cathode specific capacity (higher than 1,200 mAh/g basedon the sulfur weight, or higher than 1,000 mAh/g based on the cathodecomposite weight, including sulfur, conducting additive or substrate,and binder weights combined, but excluding the weight of a cathodecurrent collector, if present). The specific capacity is preferablyhigher than 1,400 mAh/g based on the sulfur weight alone or higher than1,200 mAh/g based on the cathode composite weight. This must beaccompanied by a high specific energy and a long and stable cycle life.

It may be noted that in most of the open literature reports (scientificpapers) and patent documents, scientists or inventors choose to expressthe cathode specific capacity based on the sulfur or lithium polysulfideweight alone (not the total cathode composite weight). Unfortunately, alarge proportion of non-active materials (such as conductive additiveand binder that are not capable of storing metal ions) is typically usedin their Li—S cells. For practical use purposes, it is more meaningfulto use the cathode composite weight-based capacity value. The instantapplication makes use of total cathode composite material weight(excluding Al foil current collector, if present) as a basis forcalculating the cathode specific capacity for our Mg—S cells.

SUMMARY OF THE INVENTION

The present invention provides a rechargeable magnesium-sulfur cellcomprising an anode active material layer, an optional anode currentcollector, a porous separator and/or an electrolyte, a metalpolysulfide-preloaded active cathode layer, and an optional cathodecurrent collector, wherein the metal polysulfide-preloaded activecathode layer (also herein referred to as an active cathode layer orpreloaded cathode layer) comprises:

-   -   A) An integral porous structure of an electronically conductive        material wherein the integral porous structure has massive        surfaces having a specific surface area greater than 100 m²/g        (Preferably >200 m²/g, more preferably >300 m²/g, further more        preferably >500 m²/g, still more preferably >700 m²/g, and most        preferably >1,000 m²/g) or has pores with a size from 1.0 nm to        100 nm and wherein multiple particles, platelets or filaments of        the conductive material form a 3-D network of        electron-conducting paths (despite not using an additional        conductive filler, such as carbon black, acetylene black, low        surface area graphite flakes with a specific surface area <100        m²/g); and    -   B) a sulfur-rich metal polysulfide, M_(x)S_(y), preloaded in        these pores or deposited on these massive surfaces, wherein x is        an integer from 1 to 3 and y is an integer from 1 to 10, and M        is a metal element selected from an alkali metal, an alkaline        metal (e.g. selected from Mg or Ca), a transition metal, a metal        from groups 13 to 17 of the periodic table, or a combination        thereof;    -   wherein the metal polysulfide is in a form of solid-state thin        coating or small particles with a thickness or diameter less        than 100 nm and occupies a weight fraction of from 1% to 99% of        the total weight of the porous structure and the metal        polysulfide combined.

In certain embodiments, the metal element M is selected from Na, K, Mg,Ca, Zn, Cu, Ti, Ni, Co, Fe, Mn, Mo, Nb, Ta, Zr, or Al. In certainpreferred embodiments, M_(x)S_(y) is selected from Li₂S, Li₂S₂, Li₂S₃,Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S, Na₂S, Na₂S₃,Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S, K₂S₂, K₂S₃, K₂S₄,K₂S₅, K₂S₆, K₂S₇, K₂S₈, K₂S₉, K₂S₁₀, MgS, MgS₂, MgS₃, MgS₄, or Mo₆S₈ (orMo₃S₄).

In this active cathode layer, the metal polysulfide is in a solid-stateform of thin coating or small particles with a thickness or diameterless than 100 nm (preferably <50 nm, more preferably <20 nm, furtherpreferably <10 nm, still more preferably <5 nm, and most preferably <3nm)) and occupies a weight fraction of from 1% to 99% (preferably atleast 50%) of the total weight of the porous structure and the metalpolysulfide combined. The weight fraction is preferably >70%, morepreferably >80%, and most preferably >90%. Preferably, the M_(x)S_(y) isloaded in these pores or deposited on these massive surfaces after (notbefore) the porous structure is made.

In an embodiment, the integral porous structure is a meso-porousstructure formed of multiple particles, platelets, or filaments of aconductive material (i.e. carbon, graphite, metal, or conductivepolymer), wherein the meso-porous structure has meso-scaled pores of2-50 nm and a specific surface area greater than 100 m²/g and whereinthe carbon, graphite, metal, or conductive polymer is selected fromchemically etched or expanded soft carbon, chemically etched or expandedhard carbon, exfoliated activated carbon, chemically etched or expandedcarbon black, chemically etched multi-walled carbon nanotube,nitrogen-doped carbon nanotube, boron-doped carbon nanotube, chemicallydoped carbon nanotube, ion-implanted carbon nanotube, chemically treatedmulti-walled carbon nanotube with an inter-planar separation no lessthan 0.4 nm, chemically expanded carbon nano-fiber, chemically activatedcarbon nano-tube, chemically treated carbon fiber, chemically activatedgraphite fiber, chemically activated carbonized polymer fiber,chemically treated coke, activated meso-phase carbon, meso-porouscarbon, electro-spun conductive nano fiber, highly separated vapor-growncarbon or graphite nano fiber, highly separated carbon nano-tube, carbonnanowire, metal nano wire, metal-coated nanowire or nano-fiber,conductive polymer-coated nanowire or nano-fiber, or a combinationthereof, and wherein the particles or filaments are optionally bonded toform said porous structure by a binder of from 0% to 30% by weight of atotal porous structure weight, not counting the metal polysulfideweight. It may be noted that un-treated carbon nano-tubes, non-separatedCNTs, and un-treated carbon black, when packed together to form anelectrode, may not have a specific surface area greater than 100 m²/m.

In another embodiment, the integral porous structure is a porousgraphene structure containing a graphene material or an exfoliatedgraphite material wherein the graphene material is selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, boron-doped graphene,nitrogen-doped graphene, chemically functionalized graphene, or acombination thereof and wherein the exfoliated graphite material isselected from exfoliated graphite worms, expanded graphite flakes, orrecompressed graphite worms or flakes, and wherein said graphenestructure comprises multiple sheets of said graphene material ormultiple flakes of said exfoliated graphite material that areintersected or interconnected to form said integral layer with orwithout a binder to bond said multiple sheets or flakes together,wherein the binder is from a resin, a conductive polymer, coal tarpitch, petroleum pitch, meso-phase pitch, coke, or a derivative thereofand occupies from 0% to 30% by weight of a total porous graphenestructure weight, not counting the metal polysulfide weight.

Prior to the battery fabrication, this preloaded cathode layer can be afree-standing layer (without being supported by a separate currentcollector) or is physically or chemically bonded to a current collectorlayer (such as a layer of Al foil).

In an embodiment, the preloaded cathode layer further comprises anelement Z or compound M_(x)Z_(y) deposited in these porous or on thesemassive surfaces wherein the element Z or M_(x)Z_(y) is mixed with themetal polysulfide or formed as discrete coating or particles having adimension less than 100 nm and Z element is selected from Sn, Sb, Bi,Se, and/or Te, and wherein x is an integer from 1 to 3, y is an integerfrom 1 to 10, and M is a metal element selected from an alkali metal, analkaline metal, a transition metal, a metal from groups 13 to 17 of theperiodic table, or a combination thereof, and the weight ratio ofZ/M_(x)S_(y) or M_(x)Z_(y)/M_(x)S_(y) is less than 1.

In the preloaded cathode layer, the metal polysulfide occupies a weightfraction of at least 70% of the total weight of the porous structure andthe metal polysulfide combined; preferably at least 80%, more preferablyat least 90%, and most preferably at least 95%. Preferably, the metalpolysulfide thickness or diameter is smaller than 10 nm, furtherpreferably smaller than 5 nm, and most preferably smaller than 3 nm.

The electrolyte is preferably selected from polymer electrolyte, polymergel electrolyte, composite electrolyte, ionic liquid electrolyte,non-aqueous liquid electrolyte, soft matter phase electrolyte,solid-state electrolyte, or a combination thereof.

Typically, a non-solid state electrolyte contains a magnesium saltdissolved in a solvent. Preferred salt/solvent combinations includeMg(AlCl₂EtBu)₂/THF, Mg(ClO₄)₂/THF, EMIC, Mg(ClO₄)₂/PC, andMg(ClO₄)₂/THF, a recrystallized Mg complex[Mg₂(μ-Cl)₃(THF)₆][HMDS-AlCl₃], (HMDS)₂Mg/diglyme, (HMDS)₂Mg/tetraglyme,and (Mg(BBu₂Ph₂)₂/THF, wherein HMDS=hexamethyldisilazide, PC=propylenecarbonate, Bu=butyl, Ph=phenyl, and THF=tetrahydrofuran. Preferably, theelectrolyte is a non-nucleophilic electrolyte. Ionic liquid, such asbis(trifluoromethanesulfonyl) imide anion-based (TFSI ⁻based), may alsobe used. Solutions of organomagnesium halides, amidomagnesium halides,and magnesium organoborates in a variety of solvents (e.g. THF, orethereal), and Grignard solutions (RMgX in ethers, R=organic alkyl oraryl group and X=halide like Cl or Br) may also be used. Usefulsolutions may be produced by reacting AlCl_((3-n))R_(n) Lewis acid withR₂Mg Lewis base in ethers (THF or glymes) at various ratios. All phenylcomplex (APC) electrolyte solutions (e.g. in THF), comprising theproducts of the reaction between PhxMgCl_((2-x)) and PhyAlCl_((3-y))(preferably PhMgCl and AlCl₃), are good electrolytes.

It may be noted that magnesium organohaloaluminate electrolytes,generated in situ from the reaction between a Lewis acid and a Lewisbase, are nucleophilic and, in prior art teaching, would be consideredun-suitable for use in a Mg—S cell. For example, a 2:1 mixture ofphenylmagnesium chloride and aluminum trichloride (AlCl₃) intetrahydrofuran (THF) would be incompatible with an electrophilic sulfurcathode. Quite unexpectedly, the implementation of the presentlyinvented active cathode layer enables this class of electrolytes to workreasonably well in the Mg—S cells. This is a great accomplishment byitself.

It may be further noted that potassium hexamethyldisilazide (KN(SiMe₃)₂)is a non-nucleophilic base and, hence, hexamethyldisilazide magnesiumchloride (HMDSMgCl) being capable of reversible Mg deposition, would bean excellent candidate electrolyte for the Mg—S cell. Unfortunately, theCoulombic efficiency, voltage stability and current density of theHMDSMgCl electrolyte are normally found to be far inferior to magnesiumorganohaloaluminate electrolytes, and, as a result, this electrolyte hasnot been accepted as a viable choice. Contrary to this belief, we havefound that this electrolyte, when used in conjunction with the presentlyinvented active cathode layer, does not have any significant issues onCoulombic efficiency, voltage stability and current density.

A special and highly advantageous feature of the inventive preloadedcathode layer is the notion that this class of metal sulfide species canbe dissolved in a solvent to form a uniform solution. This solutionreadily permeates into the pores of the porous structure and the metalsulfide, when precipitated from the solution, can deposit uniformly inthe form of a thin coating (preferably <20 nm in thickness) orultra-fine particles (preferably <20 nm in diameter) in the pores or onthe massive surfaces. This approach enables the deposition of a largeamount of metal polysulfide relative to the porous structure material(hence, achieving a high proportion of cathode active material). This isquite surprising since pure sulfur could not be uniformly deposited intoa porous structure using a solution deposition process to achieve bothhigh sulfur amount and ultra-thin coating or small particles (<20 nm).This simple solution deposition process for metal polysulfide (not forsulfur) has another advantage in that it does not require a chemicalreaction or electrochemical reaction (prior to battery fabrication),which otherwise can be more difficult to control, more tedious, and moreexpensive.

The thin coating or ultra-fine particles deposited thereon or thereinprovide ultra-short magnesium ion diffusion paths and, hence, ultra-fastreaction times for fast battery charges and discharges. This is achievedwhile, at the same time, achieving a relatively high proportion ofsulfur (the active material responsible for storing magnesium) and,thus, high specific magnesium storage capacity of the resulting cathodeactive layer in terms of high mAh/g (based on the total weight of thecathode layer, including the masses of the active material, S,supporting substrate material, optional binder resin, and optionalconductive filler combined). It is of significance to note that onemight be able to use a prior art procedure to deposit small S orpolysulfide particles, but cannot achieve a high S or metal polysulfideproportion at the same time, or to achieve a high proportion of S orpolysulfide, but only in large particles or thick coating form. Theprior art procedures have not been able to achieve both at the sametime.

It is highly advantageous to achieve a high metal polysulfide loading(hence, high sulfur loading) and yet, concurrently, form an ultra-thincoating or ultra-small diameter particles of metal polysulfide (hence,sulfur) in terms of maximizing both the energy density and power densityof a battery cell. This has not been possible with any prior art sulfuror polysulfide loading techniques. For instance, we have been able todeposit nano-scaled polysulfide particles or coating that occupy a >90%weight fraction of the cathode layer and yet maintain a coatingthickness or particle diameter <3 nm. This is quite a feat in the art ofany metal-sulfur batteries. In another example, we have achieved a >95%S loading at an average polysulfide coating thickness of 4.8-7 nm. Theseultra-thin dimensions (3-7 nm) enable facile cathode reactions andnearly perfect sulfur utilization efficiency, something that no priorworker has been able to achieve.

The strategy of pre-loading metal polysulfide, instead of pure sulfuritself, in the cathode prior to the battery cell fabrication has ahighly unexpected yet major advantage. We have surprisingly observedthat such a strategy leads to a metal-sulfur battery with a longer cyclelife. Not wishing to be limited by theory, but we think that this mightbe due to the notion that sulfur can undergo large-scale volumeexpansion when the battery is discharged. On repeatedcharges/discharges, the cathode structure can be easily damaged if anexcessively high amount of sulfur is loaded into the cathode. If insteada metal polysulfide is preloaded, the volume changes only occur to amuch smaller extent, effectively alleviating this cathode structuredamage problem.

In addition to the magnesium salts recited earlier, the electrolyte canfurther contain an alkali metal salt (lithium salt, sodium salt, and/orpotassium salt) selected from lithium perchlorate (LiClO₄), lithiumhexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithiumhexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, Lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium oxalyldifluoroborate (LiBF₂C₂O₄), Lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt, sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-metasulfonate (NaCF₃SO₃), potassium trifluoro-metasulfonate(KCF₃SO₃), bis-trifluoromethyl sulfonylimide sodium (NaN(CF₃SO₂)₂),sodium trifluoromethanesulfonimide (NaTFSI), bis-trifluoromethylsulfonylimide potassium (KN(CF₃SO₂)₂), or a combination thereof.

As examples, the solvent may be selected from ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), ethyl propionate, methyl propionate, propylene carbonate (PC),gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA),propyl formate (PF), methyl formate (MF), toluene, xylene or methylacetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC),allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane(DME), tetraethylene glycol dimethylether (TEGDME), Poly(ethyleneglycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether(DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, tetrahydrofuran(THF), room temperature ionic liquid, or a combination thereof.

Ionic liquids (ILs) are a new class of purely ionic, salt-like materialsthat are liquid at unusually low temperatures. The official definitionof ILs uses the boiling point of water as a point of reference: “Ionicliquids are ionic compounds which are liquid below 100° C.”. Aparticularly useful and scientifically interesting class of ILs is theroom temperature ionic liquid (RTIL), which refers to the salts that areliquid at room temperature or below. RTILs are also referred to asorganic liquid salts or organic molten salts. An accepted definition ofan RTIL is any salt that has a melting temperature lower than ambienttemperature. Common cations of RTILs include, but not limited to,Tetraalkylanunonium, Di-, Tri-, or Tetra-alkylimidazolium,Alkylpyridinium, Dialkylpyrrolidinium, Dialkylpiperidinium,Tetraalkylphosphonium, and Trialkylsulfonium. Common anions of RTILsinclude, but not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻,CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻,SCN⁻, SeCN⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.

In an embodiment, the anode active material layer contains an anodeactive material selected from magnesium metal, magnesium alloy (>70% byweight Mg), or magnesium compound (>70% Mg) in a chip, foil, powder,disc, platelet, fibril form or practically any shape.

In the rechargeable magnesium-sulfur cell, the binder material (ifdesired) is selected from a resin, a conductive polymer, coal tar pitch,petroleum pitch, meso-phase pitch, coke, or a derivative thereof.

In the rechargeable magnesium-sulfur cell, the cathode may furthercomprise additional sulfur, sulfur-containing molecule,sulfur-containing compound, sulfur-carbon polymer, or a combinationthereof, which is loaded before the cell is manufactured. The presentlyinvented cell provides a reversible specific capacity of typically noless than 800 mAh per gram based on the total weight of the integralcathode layer (the weights of S, graphene material, optional binder, andoptional conductive filler combined), not just based on the activematerial weight (sulfur) only. Most of the scientific papers and patentdocuments reported their sulfur cathode specific capacity data based onsulfur weight only.

More typically and preferably, the reversible specific capacity is noless than 1,000 mAh per gram and often exceeds 1,200 or even 1,500 mAhper gram of entire cathode layer. The high specific capacity of thepresently invented cathode, when in combination with a magnesium anode,leads to a cell specific energy of no less than 600 Wh/Kg based on thetotal cell weight including anode, cathode, electrolyte, separator, andcurrent collector weights combined. This specific energy value is notbased on the cathode active material weight or cathode layer weight only(as sometimes did in open literature or patent applications); instead,this is based on entire cell weight. In many cases, the cell specificenergy is higher than 700 Wh/Kg and, in some examples, exceeds 800Wh/kg.

These and other advantages and features of the present invention willbecome more transparent with the description of the following best modepractice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) SEM image of a meso-porous graphitic structure prepared byexpanding a soft carbon;

FIG. 1(B) a meso-porous graphitic structure prepared by chemicallyetching or expanding a hard carbon material;

FIG. 1(C) an expanded MCMB;

FIG. 1(D) expanded carbon fibers.

FIG. 2 Schematic of selected procedures for producing activateddisordered carbon, oxidized or fluorinated carbon (with an expandedinter-graphene spacing), expanded carbon, and activated/expanded carbonfrom disordered carbon.

FIG. 3 Schematic of selected procedures for producing activated carbonnanotubes, oxidized or fluorinated CNTs with an expanded inter-graphenespacing, and activated/expanded CNTs from multi-walled CNTs.

FIG. 4(A) Schematic of the commonly used procedures for producingexfoliated graphite worms and graphene sheets;

FIG. 4(B) Another schematic drawing to illustrate the process forproducing exfoliated graphite, expanded graphite flakes, and graphenesheets.

FIG. 5(A) SEM images of exfoliated graphite worms imaged at a lowmagnification;

FIG. 5(B) same graphite worm as in 5(A), but taken at a highermagnification;

FIG. 5(C) TEM image of single-layer graphene sheets partially stackedtogether.

FIG. 6 The specific capacities (vs. number of charge/discharge cycles)for 3 Mg—S cells: one featuring a reduced graphene oxide (RGO)-basedcathode containing solution deposited Li₂S₈ coating of the presentinvention, one containing chemically deposited sulfur in RGO, onecontaining a cathode containing RGO and Li₂S₈ that are ball-milledtogether.

FIG. 7 The specific capacities (vs. number of charge/discharge cycles)for 3 Mg—S cells: one featuring a chemically treated soft carbon(C—SC)-based cathode containing solution deposited Na₂S₈ coating of thepresent invention, one containing chemically deposited sulfur in C—SC,one containing a cathode containing C—SC and Na₂S₈ ball-milled together.

FIG. 8 Ragone plots (power density vs. energy density) of three cells: aMg—S cell featuring a MgS₆-preloaded exfoliated graphite (EG) cathodelayer, a corresponding Li—S cell, and a Mg—S cell featuring an EGcathode layer containing solution-deposited sulfur.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A. Magnesium Metal-SulfurCells

The specific capacity and specific energy of a Mg—S cell are dictated bythe actual amount of sulfur or metal polysulfide that can be implementedin the cathode active layer (relative to other non-active ingredients,such as the binder resin and conductive filler) and the utilization rateof this sulfur amount (i.e. the utilization efficiency of the cathodeactive material or the actual proportion of S or polysulfide thatactively participates in storing and releasing magnesium ions). Ahigh-capacity and high-energy Mg—S cell requires a high amount of S ormetal polysulfide in the cathode active layer (i.e. relative to theamounts of non-active materials, such as the binder resin, conductiveadditive, and other modifying or supporting materials) and a high Sutilization efficiency (little or no active material not accessible bythe electrolyte and not reachable by electrons and metal ions during thecharging/discharging operations). The present invention provides such acathode active layer and a method of producing such a cathode activelayer, which is pre-loaded with metal polysulfide (instead of S) priorto being incorporated in a Mg—S battery cell. This metal polysulfide,embedded in the cathode, can be readily converted into S during thefirst battery charging operation.

The present invention provides a rechargeable magnesium-sulfur cellfeaturing a very unique product—a metal sulfide-preloaded cathode layer(not a S-preloaded or pre-sulfurized layer). This cathode activematerial-preloaded layer (also referred to as an active cathode layer orpreloaded cathode layer) comprises:

-   -   A) An integral porous structure of an electronically conductive        material wherein the porous structure has massive surfaces        having a specific surface area greater than 100 m²/g        (preferably >300 m²/g, more preferably >500 m²/g, further        preferably >750 m²/g, and most preferably greater than 1,000        m²/g) or has pores with a size from 1.0 nm to 100 nm (preferably        from 2 nm to 50 nm) and wherein multiple particles, platelets or        filaments of this conductive material form a 3-D network of        electron-conducting paths, with or without the presence of a        conductive filler (e.g. even without the commonly used        conductive filler, such as carbon black particles or un-treated        carbon nanotubes, for the mere purpose of improving electrode        conductivity); and        -   B) a sulfur-rich metal polysulfide, M_(x)S_(y), preloaded in            these pores or deposited on these massive surfaces, wherein            x is an integer from 1 to 3 and y is an integer from 1 to            10, and M is a metal element selected from an alkali metal,            an alkaline metal (e.g. selected from Mg or Ca), a            transition metal, a metal from groups 13 to 17 of the            periodic table, or a combination thereof;

In certain embodiments, the metal element M is selected from Li, Na, K,Mg, Ca, Zn, Cu, Ti, Ni, Co, Fe, Mn, Mo, Nb, Ta, Zr, or Al. In certainpreferred embodiments, M_(x)S_(y) is selected from from Li₂S, Li₂S₂,Li₂S₃, Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S, Na₂S₂,Na₂S₃, Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S, K₂S₂,K₂S₃, K₂S₄, K₂S₅, K₂S₆, K₂S₇, K₂S₈, K₂S₉, K₂S₁₀, MgS, MgS₂, MgS₃, MgS₄,MgS₅, MgS₆, Mo₆S₈ (or Mo₃S₄), Nb₆S₈, Zr₆S₈, or Ta₆S₈, etc.

In this active cathode layer, the metal polysulfide is in a solid-stateform of thin coating or small particles with a thickness or diameterless than 100 nm (preferably <50 nm, more preferably <20 nm, furthermore preferably <10 nm, and most preferably <5 nm) and occupies a weightfraction of from 1% to 99% (preferably at least 50%) of the total weightof the porous structure and the metal polysulfide combined. The weightfraction is preferably >70%, more preferably >80%, and most preferably>90%.

Preferably, the M_(x)S_(y) is loaded in these pores or deposited onthese massive surfaces after (not before) the integral porous structureis made. This is important since metal polysulfide is not electronicallyconductive and, hence, once the otherwise conductiveparticles/platelets/filaments are coated with metal polysulfide, theresulting coated particles/platelets/filaments are no longer conducting.These coated particles/platelets/filaments, when subsequently packedinto a cathode layer, would not be electrically conductive due toelectron-conducting paths being interrupted (e.g. conductingparticle-to-particle contacts being replaced by non-conductingcoating-to-coating contact). As a result, the electrochemicalperformance of Mg—S battery becomes unsatisfactory.

In one embodiment, the integral layer of a meso-porous structure iscomposed primarily of a carbon, graphite, metal, or conductive polymerselected from chemically etched or expanded soft carbon, chemicallyetched or expanded hard carbon, exfoliated activated carbon, chemicallyetched or expanded carbon black, chemically etched multi-walled carbonnanotube, nitrogen-doped carbon nanotube, boron-doped carbon nanotube,chemically doped carbon nanotube, ion-implanted carbon nanotube,chemically treated multi-walled carbon nanotube with an inter-planarseparation no less than 0.4 nm, chemically expanded carbon nano-fiber,chemically activated carbon nano-tube, chemically treated carbon fiber,chemically activated graphite fiber, chemically activated carbonizedpolymer fiber, chemically treated coke, activated meso-phase carbon,meso-porous carbon, electro-spun conductive nano fiber, vapor-growncarbon or graphite nano fiber, carbon or graphite whisker, carbonnano-tube, carbon nanowire, metal nano wire, metal-coated nanowire ornano-fiber, conductive polymer-coated nanowire or nano-fiber, or acombination thereof. Particles and/or fibrils of this material, whenpacked into an integral electrode layer of meso-porous structure muststill exhibit a specific surface area >100 m²/g that this in directcontact with the electrolyte. The meso-pores must be accessible to theelectrolyte.

In another embodiment, the layer of integral porous structure contains agraphene material or an exfoliated graphite material, wherein thegraphene material is selected from pristine graphene, graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,boron-doped graphene, nitrogen-doped graphene, chemically functionalizedgraphene, or a combination thereof, and wherein the exfoliated graphitematerial is selected from exfoliated graphite worms, expanded graphiteflakes, or recompressed graphite worms or flakes (must still exhibit ahigh specific surface area, >>100 m²/g, accessible to electrolyte). Thegraphene structure comprises multiple sheets of a graphene material thatare intersected or interconnected to form the integral layer with orwithout a binder to bond the multiple sheets together and with orwithout a conductive filler being included in the integral porous layer.

The porous layer structure can contain, in addition to the M_(x)S_(y)discussed above, 0-49% (preferably 0-30%, more preferably 0-20%, andfurther preferably 0-10%) by weight of sulfur or other sulfur-containingcompound pre-loaded therein, based on the weights of all ingredients inthe layer. Preferably, zero (0%) sulfur or sulfur-containing compound ispre-loaded into the porous structure since this pre-loaded material, ifnot done properly, can negatively impact the subsequent pre-loadingstep.

The metal polysulfide particles or coating have a thickness or diametersmaller than 20 nm (preferably <10 nm, more preferably <5 nm, andfurther preferably <3 nm) and wherein the nano-scaled particles orcoating occupy a weight fraction of at least 70% (preferably >80%, morepreferably >90%, and most preferably >95%) based on the total weights ofthe metal sulfide particles or coating and the porous material combined.It is advantageous to deposit as much metal polysulfide as possible yetstill maintain ultra-thin thickness or diameter of coating or particles(e.g. >80% and <3 nm; >90% and <5 nm; and >95% and <10 nm, etc.).

B. Production of Various Integral Porous Structures

The following types of porous structures are found to be particularlysuitable for use to support and protect the polysulfide coating orparticles: a porous sheet, paper, web, film, fabric, non-woven, mat,aggregate, or foam of a graphene, carbon, or graphite material that hasbeen expanded, activated, chemically treated, and, in the case ofgraphene, separated or isolated.

This porous structure can contain chemically etched or expanded softcarbon, chemically etched or expanded hard carbon, exfoliated activatedcarbon, chemically etched or expanded carbon black, chemically etchedmulti-walled carbon nanotube, nitrogen-doped carbon nanotube,boron-doped carbon nanotube, chemically doped carbon nanotube,ion-implanted carbon nanotube, chemically treated multi-walled carbonnanotube with an inter-graphene planar separation no less than 0.4 nm,chemically expanded carbon nano-fiber, chemically activated or expandedcarbon nano-tube, carbon fiber, graphite fiber, carbonized polymerfiber, coke, meso-phase carbon, or a combination thereof. The expandedspacing is preferably >0.5 nm, more preferably >0.6 nm, and mostpreferably >0.8 nm.

Alternatively, the porous structure may contain a porous, electricallyconductive material selected from metal foam, carbon-coated metal foam,graphene-coated metal foam, metal web or screen, carbon-coated metal webor screen, graphene-coated metal web or screen, perforated metal sheet,carbon-coated porous metal sheet, graphene-coated porous metal sheet,metal fiber mat, carbon-coated metal-fiber mat, graphene-coatedmetal-fiber mat, metal nanowire mat, carbon-coated metal nanowire mat,graphene-coated metal nano-wire mat, surface-passivated porous metal,porous conductive polymer film, conductive polymer nano-fiber mat orpaper, conductive polymer foam, carbon foam, graphitic foam, carbonaerogel, carbon xerox gel, or a combination thereof. These porous andelectrically conductive materials are capable of accommodating sulfur intheir pores and, in many cases, capable of protecting the sulfur coatingor particles from getting dissolved in a liquid electrolyte, in additionto providing a 3-D network of electron-conducting paths. For the purposeof defining the claims, the instant cathode does not contain thoseisolated graphene sheets or platelets not supported on metal, carbon,ceramic, or polymer fibers or foams.

Further alternatively, such a porous sheet, paper, web, film, fabric,non-woven, mat, aggregate, or foam may be produced from multiple sheetsof a graphene material selected from pristine graphene, graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, or a combination thereof. Detailsare later discussed in Section C below.

Conductive polymer nano-fiber mats can be readily produced byelectro-spinning of a conductive polymer, which can be an intrinsicallyconductive (conjugate-chain) polymer or a conductive filler-filledpolymer. Electro-spinning is well-known in the art. The production ofcarbon foam, carbon aerogel, or carbon Xerox gel is also well-known inthe art.

Particularly useful metal foams include copper foam, stainless steelfoam, nickel foam, titanium foam, and aluminum foam. The fabrication ofmetal foams is well known in the art and a wide variety of metal foamsare commercially available. Preferably, the surfaces of metallic foamsare coated with a thin layer of carbon or graphene because carbon andgraphene are more electrochemically inert and will not get dissolvedduring the charge/discharge cycles of the cell. Hence, carbon-coatedmetal foam, graphene-coated metal foam, carbon-coated metal web orscreen, graphene-coated metal web or screen, carbon-coated porous metalsheet, graphene-coated porous metal sheet, carbon-coated metal-fibermat, graphene-coated metal-fiber mat, carbon-coated metal nanowire mat,and graphene-coated metal nano-wire mat are preferred current collectormaterials for use in the rechargeable lithium cell. Also particularlyuseful are carbon foam, carbon aerogel, and carbon xerox gel. Thesefoams may be reinforced with a binder resin, conductive polymer, or CNTsto make a porous structure of good structural integrity.

In one preferred embodiment, highly porous graphitic or carbonaceousmaterials may be used to make a conductive and protective backboneporous structure prior to impregnating the resulting porous structurewith metal polysulfide. In this approach, particles of these materialsmay be bonded by a binder to form a porous structure of good structuralintegrity.

In another possible route, porous graphitic or carbonaceous materialparticles, along with a resin binder, may be coated onto surfaces of ahighly porous metal framework with large pores, such as a metal foam,web, or screen, which serves as a backbone for a meso-porous structure.The combined hybrid structure is preferably very porous with a specificsurface area significantly greater than 100 m²/g.

The carbonaceous or graphitic material may be selected from chemicallytreated graphite with an inter-graphene planar separation no less than0.4 nm (preferably greater than 0.5 nm, more preferably greater than 0.6nm) which is not exfoliated, soft carbon (preferably, chemically etchedor expanded soft carbon), hard carbon (preferably, chemically etched orexpanded hard carbon), activated carbon (preferably, exfoliatedactivated carbon), carbon black (preferably, chemically etched orexpanded carbon black), chemically expanded multi-walled carbonnano-tube, chemically expanded carbon fiber or nano-fiber, or acombination thereof. These carbonaceous or graphitic materials have onething in common; they all have meso-scaled pores, enabling entry ofelectrolyte to access their interior planes of hexagonal carbon atoms.

In one preferred embodiment, the meso-porous carbonaceous material maybe produced by using the following recommended procedures:

-   -   (A) dispersing or immersing a graphitic or carbonaceous material        (e.g., powder of meso-phase carbon, meso-carbon micro bead        (MCMB), soft carbon, hard carbon, coke, polymeric carbon        (carbonized resin), activated carbon (AC), carbon black (CB),        multi-walled carbon nanotube (MWCNT), carbon nano-fiber (CNF),        carbon or graphite fiber, meso-phase pitch fiber, and the like)        in a mixture of an intercalant and/or an oxidant (e.g.,        concentrated sulfuric acid and nitric acid) and/or a        fluorinating agent to obtain a carbon intercalation compound        (CIC), graphite fluoride (GF), or chemically etched/treated        carbon material; and optionally    -   (B) exposing the resulting CIC, GF, or chemically etched/treated        carbon material to a thermal treatment, preferably in a        temperature range of 150-600° C. for a short period of time        (typically 15 to 60 seconds) to obtain expanded carbon.

Alternatively, after step (A) above, the resulting CIC, GF, orchemically etched/treated carbon material is subjected to repeatedrinsing/washing to remove excess chemical. The rinsed products are thensubjected to a drying procedure to remove water. The dried CIC, GF,chemically treated CB, chemically treated AC, chemically treated MWCNT,chemically treated CNF, chemically treated carbon/graphite/pitch fibercan be used as a cathode active material of the presently inventedhigh-capacity cell. These chemically treated carbonaceous or graphiticmaterials can be further subjected to a heat treatment at a temperaturepreferably in the range of 150-600° C. for the purposes of creatingmeso-scaled pores (2-50 nm) to enable the interior structure beingaccessed by electrolyte. It may be noted that these interior grapheneplanes remain stacked and interconnected with one another, but theabove-described chemical/thermal treatments facilitate direct access ofthese interior graphene planes by the electrolyte.

The broad array of carbonaceous materials, such as a soft carbon, hardcarbon, polymeric carbon (or carbonized resin), meso-phase carbon, coke,carbonized pitch, carbon black, activated carbon, or partiallygraphitized carbon, are commonly referred to as the disordered carbonmaterial. A disordered carbon material is typically formed of two phaseswherein a first phase is small graphite crystal(s) or small stack(s) ofgraphite planes (with typically up to 10 graphite planes or aromaticring structures overlapped together to form a small ordered domain) anda second phase is non-crystalline carbon, and wherein the first phase isdispersed in the second phase or bonded by the second phase. The secondphase is made up of mostly smaller molecules, smaller aromatic rings,defects, and amorphous carbon. Typically, the disordered carbon ishighly porous (e.g., exfoliated activated carbon), or present in anultra-fine powder form (e.g. chemically etched carbon black) havingnano-scaled features (e.g. having meso-scaled pores and, hence, a highspecific surface area).

Soft carbon refers to a carbonaceous material composed of small graphitecrystals wherein the orientations of these graphite crystals or stacksof graphene planes inside the material are conducive to further mergingof neighboring graphene sheets or further growth of these graphitecrystals or graphene stacks using a high-temperature heat treatment.This high temperature treatment is commonly referred to asgraphitization and, hence, soft carbon is said to be graphitizable.

Hard carbon refers to a carbonaceous material composed of small graphitecrystals wherein these graphite crystals or stacks of graphene planesinside the material are not oriented in a favorable directions (e.g.nearly perpendicular to each other) and, hence, are not conducive tofurther merging of neighboring graphene planes or further growth ofthese graphite crystals or graphene stacks (i.e., not graphitizable).

Carbon black (CB) (including acetylene black, AB) and activated carbon(AC) are typically composed of domains of aromatic rings or smallgraphene sheets, wherein aromatic rings or graphene sheets in adjoiningdomains are somehow connected through some chemical bonds in thedisordered phase (matrix). These carbon materials are commonly obtainedfrom thermal decomposition (heat treatment, pyrolyzation, or burning) ofhydrocarbon gases or liquids, or natural products (wood, coconut shells,etc.). These materials per se (without chemical/thermal treatments asdescribed above) are not good candidate cathode materials for thepresently invented high-capacity Li-ion cells. Hence, preferably, theyare subjected to further chemical etching or chemical/thermalexfoliation to form a meso-porous structure having a pore size in therange of 2-50 nm (preferably 2-10 nm). These meso-scaled pores enablethe liquid electrolyte to enter the pores and access the graphene planesinside individual particles of these carbonaceous materials.

The preparation of polymeric carbons by simple pyrolysis of polymers orpetroleum/coal tar pitch materials has been known for approximatelythree decades. When polymers such as polyacrylonitrile (PAN), rayon,cellulose and phenol formaldehyde were heated above 300° C. in an inertatmosphere they gradually lost most of their non-carbon contents. Theresulting structure is generally referred to as a polymeric carbon.Depending upon the heat treatment temperature (HTT) and time, polymericcarbons can be made to be insulating, semi-conducting, or conductingwith the electric conductivity range covering approximately 12 orders ofmagnitude. This wide scope of conductivity values can be furtherextended by doping the polymeric carbon with electron donors oracceptors. These characteristics uniquely qualify polymeric carbons as anovel, easy-to-process class of electro-active materials whosestructures and physical properties can be readily tailor-made.

Polymeric carbons can assume an essentially amorphous structure, or havemultiple graphite crystals or stacks of graphene planes dispersed in anamorphous carbon matrix. Depending upon the HTT used, variousproportions and sizes of graphite crystals and defects are dispersed inan amorphous matrix. Various amounts of two-dimensional condensedaromatic rings or hexagons (precursors to graphene planes) can be foundinside the microstructure of a heat treated polymer such as a PAN fiber.An appreciable amount of small-sized graphene sheets are believed toexist in PAN-based polymeric carbons treated at 300-1,000° C. Thesespecies condense into wider aromatic ring structures (larger-sizedgraphene sheets) and thicker plates (more graphene sheets stackedtogether) with a higher HTT or longer heat treatment time (e.g., >1,500°C.). These graphene platelets or stacks of graphene sheets (basalplanes) are dispersed in a non-crystalline carbon matrix. Such atwo-phase structure is a characteristic of some disordered carbonmaterial.

There are several classes of precursor materials to the disorderedcarbon materials of the instant patent application. For instance, thefirst class includes semi-crystalline PAN in a fiber form. As comparedto phenolic resin, the pyrolized PAN fiber has a higher tendency todevelop small crystallites that are dispersed in a disordered matrix.The second class, represented by phenol formaldehyde, is a moreisotropic, essentially amorphous and highly cross-linked polymer. Thethird class includes petroleum and coal tar pitch materials in bulk orfiber forms. The precursor material composition, heat treatmenttemperature (HTT), and heat treatment time (Htt) are three parametersthat govern the length, width, thickness (number of graphene planes in agraphite crystal), and chemical composition of the resulting disorderedcarbon materials.

In the present investigation, PAN fibers were subjected to oxidation at200-350° C. while under a tension, and then partial or completecarbonization at 350-1,500° C. to obtain polymeric carbons with variousnano-crystalline graphite structures (graphite crystallites). Selectedsamples of these polymeric carbons were further heat-treated at atemperature in the range of 1,500-2,000° C. to partially graphitize thematerials, but still retaining a desired amount of amorphous carbon (noless than 10%). Phenol formaldehyde resin and petroleum and coal tarpitch materials were subjected to similar heat treatments in atemperature range of 500 to 1,500° C. The disordered carbon materialsobtained from PAN fibers or phenolic resins are preferably subjected toa chemical etching/expanding treatment using a process commonly used toproduce activated carbon (e.g., treated in a KOH melt at 900° C. for 1-5hours). This chemical treatment is intended for making the disorderedcarbon meso-porous, enabling electrolyte to reach the edges or surfacesof the constituent aromatic rings after a battery cell is made. Such anarrangement enables the lithium ions in the liquid electrolyte toreadily attach onto exposed graphene planes or edges without having toundergo significant solid-state diffusion.

Certain grades of petroleum pitch or coal tar pitch may be heat-treated(typically at 250-500° C.) to obtain a liquid crystal-type, opticallyanisotropic structure commonly referred to as meso-phase. Thismeso-phase material can be extracted out of the liquid component of themixture to produce isolated meso-phase particles or spheres, which canbe further carbonized and graphitized.

In general, the cathode active material (including the porous backbonestructure and S lodged in the pores) as a whole also preferably form ameso-porous structure with a desired amount of meso-scaled pores (2-50nm, preferably 2-10 nm) to allow the entry of electrolyte. This isadvantageous because these pores enable a great amount of surface areasto be in physical contact with electrolyte and capable of capturing Sprecipitated from the electrolyte during the subsequent electrochemicaldeposition and capturing/releasing lithium (sodium or potassium) from/tothe electrolyte during subsequent battery charges/discharges. Thesesurface areas of the cathode active material as a whole are typicallyand preferably >100 m²/g, more preferably >500 m²/g, further morepreferably >1,000 m²/g, and most preferably >1,500 m²/g.

C. Production of Various Graphene-Based Integral Porous Structures

In a preferred embodiment, the graphene electrode material is selectedfrom pristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The electrode material may beselected from an exfoliated graphite material. The starting graphiticmaterial for producing any one of the above graphene or exfoliatedgraphite materials may be selected from natural graphite, artificialgraphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead,soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets (collectively, NGPs) are a new class of carbonnano material (a 2-D nano carbon) that is distinct from the 0-Dfullerene, the 1-D CNT or CNF, and the 3-D graphite. For the purpose ofdefining the claims and as is commonly understood in the art, a graphenematerial (isolated graphene sheets) is not (and does not include) acarbon nanotube (CNT) or a carbon nano-fiber (CNF).

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 4(A) and FIG. 4(B) (schematic drawings). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.4(B)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent),and another oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814 (Jun. 3,2004). Single-layer graphene can be as thin as 0.34 nm, whilemulti-layer graphene can have a thickness up to 100 nm, but moretypically less than 10 nm (commonly referred to as few-layer graphene).Multiple graphene sheets or platelets may be made into a sheet of NGPpaper using a paper-making process. This sheet of NGP paper is anexample of the porous graphene structure layer utilized in the presentlyinvented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation bas been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. 0, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid,nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≦x≦24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultra-sonic treatmentof a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 4(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 4(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 4(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications. Examples ofexfoliated graphite worms (or, simply, graphite worms) are presented inFIGS. 5(A) and 5(B).

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. A stage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG and a stage-3 GIC willhave a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs can then bebrought in contact with water or water-alcohol mixture to produceexfoliated graphite and/or separated/isolated graphene sheets.

Several techniques can be employed to fabricate a conductive layer ofporous graphene structure (a web, mat, paper, or porous film, etc.),which is a monolithic body having desired interconnected pores that areaccessible to liquid electrolyte.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is heavily re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (106 in FIG. 4(B)), whichare typically 100-500 μm thick. This conventional flexible graphite foildoes not have a specific surface area >100 m²/g. Even though theflexible graphite foil is porous, most of these pores are not accessibleto liquid electrolyte when immersed in an external electrochemicaldeposition chamber or incorporated in a lithium battery. For thepreparation of a desired layer of porous graphene structure, hecompressive stress and/or the gap between rollers can be readilyadjusted to obtain a desired layer of porous graphene structure that hasmassive graphene surfaces (having a specific surface area >100 m²/g)accessible to liquid electrolyte and available for receiving the sulfurcoating or nano particles deposited thereon.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets (NGPs) with all the graphene platelets thinner than100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 4(B)). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide may be madeinto a graphene film/paper (114 in FIG. 4(B)) using a film- orpaper-making process.

Alternatively, with a low-intensity shearing, graphite worms tend to beseparated into the so-called expanded graphite flakes (108 in FIG. 4(B)having a thickness >100 nm. These flakes can be formed into graphitepaper or mat 106 using a paper- or mat-making process, with or without aresin binder. In one preferred embodiment of the present invention, theporous web can be made by using a slurry molding or a flake/binderspraying technique. These methods can be carried out in the followingways:

As a wet process, aqueous slurry is prepared which comprises a mixtureof graphene sheets or expanded graphite flakes and, optionally, about0.1 wt. % to about 10 wt. % resin powder binder (e.g., phenolic resin).The slurry is then directed to impinge upon a sieve or screen, allowingwater to permeate through, leaving behind sheets/flakes and the binder.As a dry process, the directed sheet/flake spray-up process utilizes anair-assisted flake/binder spraying gun, which conveys flakes/sheets andan optional binder to a molding tool (e.g., a perforated metal screenshaped identical or similar to the part to be molded). Air goes throughperforations, but the solid components stay on the molding tool surface.

Each of these routes can be implemented as a continuous process. Forinstance, the process begins with pulling a substrate (porous sheet)from a roller. The moving substrate receives a stream of slurry (asdescribed in the above-described slurry molding route) from above thesubstrate. Water sieves through the porous substrate with all otheringredients (a mixture of graphene sheets or graphite flakes, optionalconductive fillers, and an optional binder) remaining on the surface ofthe substrate being moved forward to go through a compaction stage by apair of compaction rollers. Heat may be supplied to the mixture before,during, and after compaction to help cure the thermoset binder forretaining the shape of the resulting web or mat. The web or mat, withall ingredients held in place by the thermoset binder, may be storedfirst (e.g., wrapped around a roller). Similar procedures may befollowed for the case where the mixture is delivered to the surface of amoving substrate by compressed air, like in a directed fiber/binderspraying process. Air will permeate through the porous substrate withother solid ingredients trapped on the surface of the substrate, whichare conveyed forward. The subsequent operations are similar than thoseinvolved in the slurry molding route.

D. Deposition of Metal Polysulfide on Massive Conductive Surfaces or inPores of an Integral Porous Structure

Once a layer of porous structure (e.g. a porous sheet, paper, web, film,fabric, non-woven, mat, aggregate, or foam having pores of 1-100 nm insize) is prepared, this layer can be impregnated with a desired amountof sulfur-rich metal polysulfide, M_(x)S_(y), using several techniques:

The dip-coating technique is simple and effective and can be fullyautomated. In an embodiment, a proper amount of M_(x)S_(y) is dissolvedin a suitable solvent up to 0.1-10% by weight (typically <5%) to form asolution. A porous film (paper, web, fabric, foam, etc.) can be fed froma feeder roller and immersed into a bath containing such solution andemerged from this path, allowing the solvent to be removed before theimpregnated porous film is wound on a winding roller. With a proper poresize range (preferably 2-50 nm) and surface chemical state of theconducting material (e.g. graphene surface, exfoliated graphite flakesurface, etc.), species of M_(x)S_(y) readily migrates into the poresand deposit, as a coating or nano particles, onto pore internal wallsurfaces (or internal graphene domain surfaces), or simply precipitatesout as nano M_(x)S_(y) particles residing in the pores of the porousstructure. This is a roll-to-roll or reel-to-reel process and is highlyscalable. In other words, the active cathode layer can be mass producedcost-effectively.

The liquid dispensing and coating technique is also simple andeffective, and can be automated as well. Again, a layer of porousstructure can be fed from a feeder roller and collected on a windingroller. Between these two ends, a solution or suspension (containingM_(x)S_(y) dissolved/dispersed in a liquid solvent) is dispensed anddeposited on one or both surfaces of a porous structure. Heating and/ordrying provisions are also installed to help remove the solvent,allowing the M_(x)S_(y) species to permeate into the porous structureand precipitate out as a nano coating or nano particles. A broad arrayof dispensing/depositing techniques can be used; e.g. spraying (aerosolspraying, ultrasonic spraying, compressed air-driven spraying, etc.),printing (inkjet printing, screen printing, etc.), and coating (slot-diecoating, roller coating, etc.). This is a highly scalable, roll-to-rollprocess.

The processing conditions can be readily adjusted to deposit M_(x)S_(y)particles or coating that have a thickness or diameter smaller than 20nm (preferably <10 nm, more preferably <5 nm, and further preferably <3nm). The resulting nano-scaled metal polysulfide particles or coatingoccupy a weight fraction of from 1% to 99%, but preferably at least 50%(preferably >70%, further preferably 80%, more preferably >90%, and mostpreferably >95%) based on the total weights of the sulfur particles orcoating and the graphene material combined.

A range of polysulfide, M_(x)S_(y), can be selected, wherein x is aninteger from 1 to 3 and y is an integer from 1 to 10, and M is a metalelement selected from an alkali metal, an alkaline metal (preferablyselected from Mg or Ca), a transition metal, a metal from groups 13 to17 of the periodic table, or a combination thereof. It is essential thatthere is at least a solvent that can readily dissolve the polysulfideM_(x)S_(y). It may be noted that this M does not have to be Mg eventhough this M_(x)S_(y) is intended to be utilized as a precursor to thecathode active material of the Mg—S cell. We have surprisingly observedthat the presence of a non-Mg metal element at the cathode can alter thechemical nature of electrolyte relative to sulfur in a favorable manner;i.e. making the electrolyte more compatible with the sulfur cathode.This is a highly desirable and unexpected feature. In this context, themetal element M is preferably selected from Li, Na, K, Ca, Zn, Cu, Ti,Ni, Co, Fe, Mn, Mo, Nb, Ta, Zr, or Al.

In certain preferred embodiments, M_(x)S_(y) is selected from Li₂S,Li₂S₂, Li₂S₃, Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S,Na₂S₂, Na₂S₃, Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S,K₂S₂, K₂S₃, K₂S₄, K₂S₅, K₂S₆, K₂S₇, K₂S₈, K₂S₉, K₂S₁₀, MgS, MgS₂, MgS₃,MgS₄, MgS₅, MgS₆, or Mo₆S₈ (or Mo₃S₄). Depending upon the intended typeof M_(x)S_(y) used, the solvent may be selected from 1,3-dioxolane(DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether(TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethyleneglycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone,sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC),methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate,methyl propionate, propylene carbonate (PC), gamma-butyrolactone (γ-BL),acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methylformate (MF), toluene, xylene, methyl acetate (MA), fluoroethylenecarbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), ahydrofluoroether, a room temperature ionic liquid solvent, or acombination thereof. The M_(x)S_(y) deposition is conducted before thecathode active layer is incorporated into an intended Mg—S cell.

After an extensive and in-depth research effort, we have come to realizethat such a M_(x)S_(y) pre-loading strategy surprisingly solves severalmost critical issues associated with current Mg—S cells. For instance,this method enables the metal sulfide to be deposited in a thin coatingor ultra-fine particle form, thus, providing ultra-short diffusion pathsfor Mg ions and, hence, ultra-fast reaction times for fast batterycharges and discharges. This is achieved while maintaining a relativelyhigh proportion of metal sulfide, which is later converted into sulfurin the battery cell. Since sulfur is the active material responsible forstoring Mg, this high loading of M_(x)S_(y) implies a high specific Mgstorage capacity of the resulting cathode active layer in terms ofmAh/g, based on the total weight of the cathode layer, including themasses of the active material, supporting conductive material such asgraphene sheets, optional binder resin, and optional conductive filler).

It is of significance to note that one might be able to use a prior artprocedure to deposit small particles of S or select metal polysulfide,but not a high S or polysulfide proportion, or to achieve a highproportion but only in large particles or thick film form. But, theprior art procedures have not been able to achieve both high S orpolysulfide proportion and ultra-thin coating/particles at the sametime. It is highly advantageous to obtain a high metal polysulfideloading and yet, concurrently, maintaining an ultra-thin/smallthickness/diameter of polysulfide for significantly enhanced energydensity and power density. This has not been possible with any prior artsulfur loading techniques (not even for Li—S cells, let alone for Mg—Scells). For instance, we have been able to deposit nano-scaled metalpolysulfide particles or coating that occupy a >90% weight fraction ofthe cathode layer and yet maintaining a coating thickness or particlediameter <3 nm. This is quite a feat in the art of Mg—S cells (in theart of lithium-sulfur batteries as well). As another example, we haveachieved a >95% S loading at an average polysulfide coating thickness of4.0-6 nm.

We have observed that the key to achieving a high active materialutilization efficiency is minimizing the S or metal polysulfide coatingor particle size and is independent of the amount of S or metalpolysulfide loaded into the cathode provided the coating or particlethickness/diameter is small enough (e.g. <10 nm, or even better if <5nm). The problem here is that it has not been previously possible tomaintain a thin S or metal polysulfide coating or small particle size ifS or metal polysulfide is higher than 50% by weight. Here we havefurther surprisingly observed that the key to enabling a high specificcapacity at the cathode under high charge/discharge rate conditions isto maintain a high S or metal polysulfide loading and still keep thecoating or particle size as small as possible, and this is accomplishedby using the presently invented metal polysulfide pre-loading method.

The electrons coming from or going out through the external load orcircuit must go through the conductive additives (in a conventionalsulfur cathode) or a conductive framework (e.g. conductive meso-porousstructure as herein disclosed) to reach the cathode active material.Since the cathode active material (e.g. sulfur or metal polysulfide) isa poor electronic conductor, the active material particle or coatingmust be as thin as possible to reduce the required electron traveldistance.

Furthermore, the cathode in a conventional Li—S cell typically has lessthan 70% by weight of sulfur in a composite cathode composed of sulfurand the conductive additive/support. Even when the sulfur content in theprior art composite cathode reaches or exceeds 70% by weight, thespecific capacity of the composite cathode is typically significantlylower than what is expected based on theoretical predictions. Forinstance, the theoretical specific capacity of sulfur is 1,675 mAh/g. Acomposite cathode composed of 70% sulfur (S) and 30% carbon black (CB),without any binder, should be capable of storing up to 1,675×70%=1,172mAh/g. Unfortunately, the observed specific capacity is typically lessthan 879 mAh/g (<75% of S being utilized) and often less than 586 mAh/g(or <50% in this example) of what could be achieved. In other words, theactive material (S) utilization rate is typically less than 75% (or even<50%). This has been a major issue in the art of Li—S cells (and in thefield of Mg—S cells as well) and there has been no effective solution tothis problem. Most surprisingly, the implementation of a porousstructure as a conductive supporting material for metal polysulfide hasmade it possible to achieve an active material utilization rate oftypically >>80%, more often greater than 90%, and, in many cases, closeto 95%-99%.

Still another unexpected result of the instant invention is theobservation that thinner polysulfide coating leads to more stablecharge/discharge cycling with significantly reduced shuttling effectthat has been a long-standing impediment to full commercialization ofLi—S batteries. Mg—S cells also have the same shuttling effect issue. Weovercome this problem yet, at the same time, achieving a high specificcapacity. In all prior art Li—S cells, a higher S loading leads to afaster capacity decay. The shuttling effect is related to the tendencyfor sulfur or metal polysulfide that forms at the cathode to getdissolved in the solvent and for the dissolved polysulfide species tomigrate from the cathode to the anode, where they irreversibly reactwith anode materials to form species that prevent sulfide from returningback to the cathode during the subsequent discharge operation of thecell (the detrimental shuttling effect). It seems that the presence ofmassive carbon or graphene surfaces have been able to prevent or reducesuch a dissolution and migration issue.

Further significantly, we have unexpectedly discovered that aM_(x)S_(y)-preloaded cathode layer is more robust than a S-preloadedcathode layer in terms of maintaining the specific capacity of thecathode. This is likely due to the notion that a M_(x)S_(y)-preloadedcathode layer has already naturally built in some expanded volume andhence is less prone or more resistant to sulfur volume expansion-induceddamage upon repeated charges/discharges.

In one embodiment (not preferably), the cathode layer may be pre-loadedwith up to 30% (preferably <15% and more preferably <10%) of an activematerial (S or lithium polysulfide) prior to the active cathode layerfabrication. In yet another embodiment, the cathode layer can contain aconductive filler, such as carbon black (CB), acetylene black (AB),graphite particles, activated carbon, meso-porous carbon, meso-carbonmicro bead (MCMB), carbon nano-tube (CNT), carbon nano-fiber (CNF),carbon fiber, or a combination thereof. These materials (notmeso-porous) are merely for use as a conductive filler, not as a supportfor polysulfide.

E. Electrolyte and Anode

After this metal polysulfide-preloaded active cathode layer is made,this cathode layer (along with an optional cathode current collector)can be combined with an anode active material layer, an optional anodecurrent collector, a porous separator and/or an electrolyte to form aMg—S cell.

The anode active material layer may contain a layer of anode activematerial that contains at least 70% by weight Mg. This can include Mgmetal (>99.9% Mg) or a Mg alloy or compound that contains from 70% to99.9% Mg. This active material can be in any shape, but preferably inthe form of a sheet/film of Mg or Mg alloy, or in a form of multiplefine particles, discs, platelets, chips, or filaments that are packedtogether to form a layer. This active material may be supported bymassive surfaces of a conductive material having a high specific surfacearea (e.g. >100 m²/g).

The electrolyte may be selected from polymer electrolyte, polymer gelelectrolyte, composite electrolyte, ionic liquid electrolyte,non-aqueous liquid electrolyte, soft matter phase electrolyte,solid-state electrolyte, or a combination thereof. Ionic liquid, such asbis(trifluoromethanesulfonyl) imide anion-based (TFSI ⁻based), may alsobe used.

Typically, a non-solid state electrolyte contains a magnesium saltdissolved in a solvent. Preferred salt/solvent combinations includeMg(AlCl₂EtBu)₂/THF, Mg(ClO₄)₂/THF, EMIC, Mg(ClO₄)₂/PC, andMg(ClO₄)₂/THF, a recrystallized Mg complex[Mg₂(μ-Cl)₃(THF)₆][HMDS-AlCl₃], (HMDS)₂Mg/diglyme, (HMDS)₂Mg/tetraglyme,and (Mg(BBu₂Ph₂)₂/THF, wherein HMDS=hexamethyldisilazide, PC=propylenecarbonate, Bu=butyl, Ph=phenyl, and THF=tetrahydrofuran. Preferably, theelectrolyte is a non-nucleophilic electrolyte.

Solutions of organomagnesium halides, amidomagnesium halides, andmagnesium organoborates in a variety of solvents (e.g. THF, orethereal), and Grignard solutions (RMgX in ethers, R=organic alkyl oraryl group and X=halide like Cl or Br) may also be used. Usefulsolutions may be produced by reacting AlCl_((3-n))R_(n) Lewis acid withR₂Mg Lewis base in ethers (THF or glymes) at various ratios. All phenylcomplex (APC) electrolyte solutions (e.g. in THF), comprising theproducts of the reaction between PhxMgCl_((2-x)) and PhyAlCl_((3-y))(preferably PhMgCl and AlCl₃), are good electrolytes.

Magnesium organohaloaluminate electrolytes, produced in situ from thereaction between a Lewis acid and a Lewis base, are nucleophilic and, inprior art teaching, would be considered un-suitable for use in a Mg—Scell. For example, a 2:1 mixture of phenylmagnesium chloride andaluminum trichloride (AlCl₃) in tetrahydrofuran (THF) would beincompatible with an electrophilic sulfur cathode. Quite unexpectedly,the implementation of the presently invented active cathode layerenables this class of electrolytes to work reasonably well in the Mg—Scells. Not wishing to be bound by theory, but it seems that the presentstrategy somehow is effective in preventing this class of electrolytesfrom directly reacting with sulfur to form phenyl disulfide and biphenylsulfide.

It may be noted that potassium hexamethyldisilazide (KN(SiMe₃)₂) is anon-nucleophilic base and, hence, hexamethyldisilazide magnesiumchloride (HMDSMgCl) being capable of reversible Mg deposition, would bean excellent candidate electrolyte for the Mg—S cell. Unfortunately, theCoulombic efficiency, voltage stability and current density of theHMDSMgCl electrolyte are normally found to be far inferior to magnesiumorganohaloaluminate electrolytes, and, as a result, this electrolyte hasnot been accepted as a viable choice. Quite unexpectedly, we have foundthat this electrolyte, when used in conjunction with the presentlyinvented active cathode layer, does not have any significant issues onCoulombic efficiency, voltage stability and current density.

A certain amount of Li-, Na- or other metal salts can be added intothese electrolytes designed presumably for Mg—S cells. This second salt,as a modifier, should preferably less than 50% by weight of the totalsalt weight; more preferably less than 30% and most preferably less than10%. This second or modifying salt may be may be selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiASF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂], lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI), orbis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂). Theelectrolyte may contain an ionic liquid.

A well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

The following examples are presented primarily for the purpose ofillustrating the best mode practice of the present invention and shouldnot be construed as limiting the scope of the present invention.

Example 1 Meso-porous Soft Carbon as a Supporting and ProtectiveBackbone for Metal Polysulfide

Chemically etched or expanded soft carbon was prepared fromheat-treating a liquid crystalline aromatic resin (50/50 mixture ofanthracene and pyrene) at 200° C. for 1 hour. The resin was ground witha mortar, and calcined at 900° C. for 2 h in a N₂ atmosphere to preparethe graphitizable carbon or soft carbon. The resulting soft carbon wasmixed with small tablets of KOH (four-fold weight) in an alumina meltingpot. Subsequently, the soft carbon containing KOH was heated at 750° C.for 2 h in N₂. Upon cooling, the alkali-rich residual carbon was washedwith hot water until the outlet water reached a pH value of 7. Theresulting chemically etched or expanded soft carbon was dried by heatingat 60° C. in a vacuum for 24 hours. This material can be used in boththe anode and cathode due to its high specific surface area and itsability to capture and store Mg atoms on its surfaces. These surfaces(inside pores) were also found to be particularly suitable forsupporting metal polysulfide nano coating or nano particles.

Example 2 Expanded “Activated Carbon” (E-AC) as a Supporting andProtective Porous Backbone for Metal Polysulfide

Activated carbon (AC, from Ashbury Carbon Co.) was treated with an acidsolution (sulfuric acid, nitric acid, and potassium permanganate at aratio of 4:1:0.05) for 24 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The treated AC wasrepeatedly washed in a 5% solution of HCl to remove most of the sulphateions. The sample was then washed repeatedly with deionized water untilthe pH of the filtrate was neutral. The slurry was then dried in avacuum oven pre-set at 70° C. for 24 hours. The dried sample was thenplaced in a tube furnace at 1,050° C. for 2 minutes to obtain expandedAC. This material can be used in both the anode and cathode of a Mg celldue to its high specific surface area and ability to capture and storeMg and/or S atoms on its surfaces. These surfaces were also found to beparticularly suitable for supporting nano metal polysulfide inmeso-pores.

Example 3 Chemically Treated (Expanded) Needle Coke as a Supporting andProtective Porous Backbone for Metal Polysulfide

Anisotropic needle coke has a fully developed needle-shape texture ofoptical anisotropy. Volatile species of the raw coke was estimated to bearound 5 wt. %. Activation was carried out using KOH in a reactionapparatus that consisted of a stainless steel tube and a nickel sampleholder. KOH activation was carried out at 800° C. for 2 h under Ar flow.The coke/KOH ratio was varied between 1/1 and 1/4. Upon cooling, thealkali-rich coke was washed with hot water until the outlet waterreached a pH value of 7. The resulting chemically etched or expandedcoke was dried by heating at 60° C. in a vacuum for 24 hours. Thetreated coke is highly porous, having a pore size range of approximately1-85 nm and being suitable for entry of metal polysulfide solution anddeposition of metal polysulfide.

Example 4 Chemically Treated (Expanded) Petroleum Pitch-Derived HardCarbon as a Supporting and Protective Porous Backbone

A pitch sample (A-500 from Ashland Chemical Co.) was carbonized in atube furnace at 900° C. for 2 hours, followed by further carbonizationat 1,200° C. for 4 hours. KOH activation was carried out at 800° C. for2 h under Ar flow to open up the internal structure of pitch-based hardcarbon particles. The hard carbon-based porous structure was found tohave a pore size range of 3-100 nm (mostly <50 nm) and to beparticularly suitable for supporting and protecting metal polysulfidelodged therein.

Example 5 Chemically Activated Meso-phase Carbon and Production ofFluorinated Carbon as a Supporting and Protective Porous Backbone

Meso-carbon carbon particles (un-graphitized MCMBs) were supplied fromChina Steel Chemical Co. This material has a density of about 2.2 g/cm³with a median particle size of about 16 μm. This batch of meso-phasecarbon was divided into two samples. One sample was immersed in K₂CO₃ at900° C. for 1 h to form chemically activated meso-carbon. The chemicallyactivated meso-phase carbons showed a BET specific surface area of 1,420m²/g. This material can be used in both the anode and cathode due to itshigh specific surface area and ability to capture and store metal atomson its surfaces. These surfaces were found to be suitable for supportingand protecting metal polysulfide nano coating or particles as well.

Example 6 Graphitic Fibrils from Pitch-based Carbon Fibers for Forming aPorous Backbone

Fifty grams of pitch-based graphite fibers were intercalated with amixture of sulfuric acid, nitric acid, and potassium permanganate at aweight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for 24hours. Upon completion of the intercalation reaction, the mixture waspoured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 5. The dried sample was then exposed to aheat shock treatment at 950° C. for 45 seconds. The sample was thensubmitted to a mechanical shearing treatment in a Cowles (arotating-blade dissolver/disperser) for 10 minutes. The resultinggraphitic fibrils were examined using SEM and TEM and their length anddiameter were measured. Graphitic fibrils, alone or in combination withanother particulate carbon/graphite material, can be packed into ameso-porous structure (mat or paper) for supporting metal polysulfide.

Example 7 Expanded Multi-walled Carbon Nanotubes (MWCNTs) as aSupporting and Protective Porous Backbone

Fifty grams of MWCNTs were chemically treated (intercalated and/oroxidized) with a mixture of sulfuric acid, nitric acid, and potassiumpermanganate at a weight ratio of 4:1:0.05 (graphite-to-intercalateratio of 1:3) for 48 hours. Upon completion of the intercalationreaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 5. The dried samplewas then exposed to a heat shock treatment at 950° C. for 45 seconds.Expanded MWCNTs, alone or in combination with another particulatecarbon/graphite material, can be packed into a meso-porous structure forsupporting metal polysulfide.

Example 8 Preparation of Graphene Oxide (GO) and Reduced GO Nano Sheetsfrom Natural Graphite Powder and their Paper/Mats (Layers of PorousGraphene Structure)

Natural graphite from Huadong Graphite Co. (Qingdao, China) was used asthe starting material. GO was obtained by following the well-knownmodified Hummers method, which involved two oxidation stages. In atypical procedure, the first oxidation was achieved in the followingconditions: 1100 mg of graphite was placed in a 1000 mL boiling flask.Then, 20 g of K₂S₂O₈, 20 g of P₂O₅, and 400 mL of a concentrated aqueoussolution of H₂SO₄ (96%) were added in the flask. The mixture was heatedunder reflux for 6 hours and then let without disturbing for 20 hours atroom temperature. Oxidized graphite was filtered and rinsed withabundant distilled water until neutral pH. A wet cake-like material wasrecovered at the end of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt. %) anddeoxycholate sodium (50 wt. %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt. %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt. % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt. % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt. % aqueous solutionof N₂H₄ (hydrazine) and 70 mL of a 28 wt. % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction. Thesesuspensions (GO in water and RGO in surfactant water) were then filteredthrough a vacuum-assisted membrane filtration apparatus to obtain GO andRGO paper or mat. These porous paper/mat structures can be used toaccommodate metal polysulfide to form a cathode layer.

Example 9 Preparation of Discrete Functionalized GO Sheets from GraphiteFibers and Porous Films of Chemically Functionalized GO

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a pot of slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. Ammonia waterwas added to one pot of the resulting suspension, which wasultrasonicated for another hour to produce NH₂-functionalized grapheneoxide (f-GO). The GO sheets and functionalized GO sheets were separatelydiluted to a weight fraction of 5% and the suspensions were allowed tostay in the container without any mechanical disturbance for 2 days,forming liquid crystalline phase in the water-alcohol liquid whenalcohol is being vaporized at 80° C.

The resulting suspensions containing GO or f-GO liquid crystals werethen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. The resulting GO or f-GOcoating films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 500 μm. The resulting GO film was thensubjected to heat treatments that involve an initial thermal reductiontemperature of 80-350° C. for 8 hours, followed by heat-treating at asecond temperature of 1,500-2,850° C. for different specimens to obtainvarious porous graphitic films. These porous films can be used toaccommodate metal polysulfide to form a cathode layer.

Example 10 Preparation of Single-Layer Graphene Sheets and PorousGraphene Mats from Meso-Carbon Micro-Beads (MCMBs)

Meso-carbon micro-beads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. In one example, MCMB (10grams) were intercalated with an acid solution (sulfuric acid, nitricacid, and potassium permanganate at a ratio of 4:1:0.05) for 48-96hours. Upon completion of the reaction, the mixture was poured intodeionized water and filtered. The intercalated MCMBs were repeatedlywashed in a 5% solution of HCl to remove most of the sulphate ions. Thesample was then washed repeatedly with deionized water until the pH ofthe filtrate was no less than 4.5. The slurry was then subjectedultrasonication for 10-100 minutes to fully exfoliate and separate GOsheets. TEM and atomic force microscopic studies indicate that most ofthe GO sheets were single-layer graphene when the oxidation treatmentexceeded 72 hours, and 2- or 3-layer graphene when the oxidation timewas from 48 to 72 hours. The GO sheets contain oxygen proportion ofapproximately 35%-47% by weight for oxidation treatment times of 48-96hours.

The suspension was then diluted to approximately 0.5% by weight in acontainer and was allowed to age therein without mechanical disturbance.The suspension was then slightly heated (to 65° C.) to vaporize thewater under a vacuum-pumping condition. The formation of liquidcrystalline phase became more apparent as water was removed and the GOconcentration was increased. The final concentration in this sample wasset at 4% by weight. The dispersion containing liquid crystals of GOsheets was then cast onto a glass surface using a doctor's blade toexert shear stresses, inducing GO sheet orientations. The resulting GOfilms, after removal of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm. The resulting GO compact was then subjectedto heat treatments to produce porous structures. These treatmentstypically involve an initial thermal reduction temperature of 80-500° C.for 1-5 hours, followed by heat-treating at a second temperature of1,500-2,850° C. These porous films can be used to accommodate metalpolysulfide to form a cathode layer.

Example 11 Preparation of Pristine Graphene Sheets/Platelets (0% Oxygen)and the Effect of Pristine Graphene Sheets

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free) can lead to a HOGF having a higher thermalconductivity. Pristine graphene sheets were produced by using the directultrasonication or liquid-phase production process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free. Thesuspension was then filtered via vacuum-assisted filtration to obtainporous graphene paper structures. These porous structures can be used toaccommodate metal polysulfide to form a cathode layer. All porousstructures can also be used to support Mg metal or alloy in the anode.

Example 12 Preparation of Graphene Fluoride Nano Sheets and PorousGraphene Structure from these Sheets

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion of few-layer graphenefluoride, but longer sonication times ensured the production of mostlysingle-layer graphene fluoride sheets. Some of these suspension sampleswere subjected to vacuum oven drying to recover separated graphenefluoride sheets. These graphene fluoride sheets were then added into apolymer-solvent or monomer-solvent solution to form a suspension.Various polymers or monomers (or oligomers) were utilized as theprecursor film materials for subsequent carbonization and graphitizationtreatments.

Upon casting on a glass surface with the solvent removed, the dispersionbecame a brownish film formed on the glass surface. When theseGF-reinforced polymer films were heat-treated, fluorine and othernon-carbon elements were released as gases that generated pores in thefilm. The resulting porous graphitic films had physical densities from0.33 to 1.22 g/cm³. These porous graphitic films were then roll-pressedto obtain graphitic films (porous graphene structures) having a densityfrom 0.8 to 1.5 g/cm³. These porous films can be used to accommodatemetal polysulfide to form a cathode layer.

Example 13 Preparation of Nitrogenataed Graphene Nano Sheets and PorousGraphene Structures

Graphene oxide (GO), synthesized in Example 1, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 Wt % respectively asfound by elemental analysis. These nitrogenataed graphene sheets remaindispersible in water. Two types of dispersions were then prepared. Oneinvolved adding water-soluble polymer (e.g. polyethylene oxide) into thenitrogenated graphene sheet-water dispersion to produce a water-basedsuspension. The other involved drying the nitrogenated graphenesheet-water dispersion to recover nitrogenated graphene sheets, whichwere then added into precursor polymer-solvent solutions to obtainorganic solvent-based suspensions.

The resulting suspensions were then cast, dried, carbonized andgraphitized to produce porous graphene structures. The carbonizationtemperatures for comparative samples are 900-1,350° C. Thegraphitization temperatures are from 2,200° C. to 2,950° C. These porousstructures can be used to accommodate metal polysulfide to form acathode layer.

Example 14 Exfoliated Graphite Worms from Natural Graphite

Natural graphite, nominally sized at 45 μm, provided by Asbury Carbons(405 Old Main St., Asbury, N.J. 08802, USA) was milled to reduce thesize to approximately 14 μm. The chemicals used in the present study,including fuming nitric acid (>90%), sulfuric acid (95-98%), potassiumchlorate (98%), and hydrochloric acid (37%), were purchased fromSigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged withsulfuric acid (360 mL) and nitric acid (180 mL) and cooled by immersionin an ice bath. The acid mixture was stirred and allowed to cool for 15min, and graphite (20 g) was added under vigorous stirring to avoidagglomeration. After the graphite powder was well dispersed, potassiumchlorate (110 g) was added slowly over 15 min to avoid sudden increasesin temperature. The reaction flask was loosely capped to allow evolutionof gas from the reaction mixture, which was stirred for 48 hours at roomtemperature. On completion of the reaction, the mixture was poured into8 L of deionized water and filtered. The slurry was spray-dried torecover an expandable graphite sample. The dried, expandable graphitewas quickly placed in a tube furnace preheated to 1,000° C. and allowedto stay inside a quartz tube for approximately 40 seconds to obtainexfoliated graphite worms. Some of the graphite forms were thenroll-pressed to obtain samples of re-compressed exfoliated graphitehaving a range of physical densities (e.g. 0.3 to 1.2 g/cm³). Some ofthe graphite worms were subjected to low-intensity sonication to produceexpanded graphite flakes. These expanded graphite flakes were then madeinto a porous paper form using the vacuum-assisted filtration technique.These porous structures can be used to accommodate metal polysulfide toform a cathode layer.

Example 15 Exfoliated Graphite Worms from Various Synthetic GraphiteParticles or Fibers

Additional exfoliated graphite worms were prepared according to the sameprocedure described in Example 1, but the starting graphite materialswere graphite fiber (Amoco P-100 graphitized carbon fiber), graphiticcarbon nano-fiber (Pyrograph-III from Applied Science, Inc., Cedarville,Ohio), spheroidal graphite (HuaDong Graphite, QinDao, China), andmeso-carbon micro-beads (MCMBs) (China Steel Chemical Co., Taiwan),respectively. These four types of laminar graphite materials wereintercalated and exfoliated under similar conditions as used for Example1 for different periods of time, from 24 hours to 96 hours.

Some amount of the graphite forms was then roll-pressed to obtainsamples of re-compressed exfoliated graphite having a range of physicaldensities (e.g. 0.3 to 1.2 g/cm³). A second amount of the graphite wormswas subjected to low-intensity sonication to produce expanded graphiteflakes. These expanded graphite flakes were then made into a paper form(the porous structure) using the vacuum-assisted filtration technique.These porous structures can be used to accommodate metal polysulfide toform a cathode layer.

Example 16 Exfoliated Graphite Worms from Natural Graphite Using HummersMethod

Additional graphite intercalation compound (GIC) was prepared byintercalation and oxidation of natural graphite flakes (original size of200 mesh, from Huadong Graphite Co., Pingdu, China, milled toapproximately 15 μm) with sulfuric acid, sodium nitrate, and potassiumpermanganate according to the method of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite,we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams ofpotassium permanganate, and 0.5 grams of sodium nitrate. The graphiteflakes were immersed in the mixture solution and the reaction time wasapproximately three hours at 30° C. It is important to caution thatpotassium permanganate should be gradually added to sulfuric acid in awell-controlled manner to avoid overheat and other safety issues. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed repeatedly with deionized wateruntil the pH of the filtrate was approximately 5. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theresulting GIC was exposed to a temperature of 1,050° C. for 35 secondsin a quartz tube filled with nitrogen gas to obtain worms of exfoliatedgraphite flakes.

Some of the graphite forms were then roll-pressed to obtain samples ofre-compressed exfoliated graphite having a range of physical densities(e.g. 0.3 to 1.2 g/cm³). Some of the graphite worms were subjected tolow-intensity sonication to produce expanded graphite flakes. Theseexpanded graphite flakes were then made into a porous paper form usingthe vacuum-assisted filtration technique. These porous structures can beused to accommodate metal polysulfide to form a cathode layer. Allporous structures can also be used to support Mg metal or alloy in theanode

Example 17 Conductive Web of Filaments from Electro-spun PAA Fibrils asa Supporting Layer for Both an Anode and a Cathode

Poly (amic acid) (PAA) precursors for spinning were prepared bycopolymerizing of pyromellitic dianhydride (Aldrich) and4,4′-oxydianiline (Aldrich) in a mixed solvent oftetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solutionwas spun into fiber web using an electrostatic spinning apparatus. Theapparatus consisted of a 15 kV d.c. power supply equipped with thepositively charged capillary from which the polymer solution wasextruded, and a negatively charged drum for collecting the fibers.Solvent removal and imidization from PAA were performed concurrently bystepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide(PI) web samples were carbonized at 1,000° C. to obtain carbonizednano-fibers with an average fibril diameter of 67 nm. Such a web can beused as a conductive substrate for an anode active material. We observethat the implementation of a network of conductive nano-filaments at theanode of a Li—S cell can effectively suppress the initiation and growthof lithium dendrites that otherwise could lead to internal shorting.Carbonized version of PI nano-fibers can be formed into a meso-porousmat for supporting metal polysulfide in the cathode, or Mg metal in theanode.

Example 18 Deposition of Metal Polysulfide in Various Meso-Porous Websor Paper Structures Prepared in Previous Examples for Mg—S Batteries

The deposition of metal polysulfide was conducted before the cathodeactive layer was incorporated into a Mg—S battery cell. In a typicalprocedure, a metal polysulfide (M_(x)S_(y)) is dissolved in a solvent(e.g. mixture of DOL/DME in a volume ratio from 1:3 to 3:1) to form anelectrolyte solution. Several types of metal polysulfide materials arecommercially available. A wide variety of solvents can be utilized forthis purpose and there is no theoretical limit to what type of solventscan be used; any solvent can be used provided that there is somesolubility of the metal polysulfide in this desired solvent. A greatersolubility would mean a larger amount of polysulfide can be precipitatedout from the electrolyte solution and deposited in the porous structure.

Those commercially unavailable metal polysulfide materials can bereadily prepared in a lab setting. As a series of examples, lithiumpolysulfide (Li_(x)S_(y)) and sodium polysulfide (Na_(x)S_(y)) withdesired x and y values (e.g. x=2, and y=6-10) dissolved in solvent wereprepared by chemically reacting stoichiometric amounts of sulfur andLi₂S or Na₂S in polysulfide free electrolyte of 0.5 M LiTFSI+0.2 M LiNO₃(or 0.5 M NaTFSI+0.2 M NaNO₃) in DOL/DME (1:1, v:v). The electrolyte wasstirred at 75° C. for 3-7 hours and then at room temperature for 48hours. The resulting electrolytes contain different Li_(x)S_(y) orNa_(x)S_(y) species (e.g. x=2, and y=6-10, depending upon reaction timesand temperatures), which are intended for use as a sulfur source in aMg—S battery cell.

Several methods were utilized to introduce polysulfide-solvent solutioninto the pores of the conductive porous structure. One method entaileddrawing a desired amount of solution into a syringe, which was thendischarged and dispensed onto the porous structure. In most cases, thesolution naturally permeates into the pores. Another method involvedusing a lab-scale liquid sprayer to spray the solution over the porousstructure. Yet another method included dipping the entire porousstructure into the solution for a desired period of time. In allmethods, precipitation of metal polysulfide occurred upon removal of thesolvent. This drying procedure allows the precipitated polysulfide todeposit onto the internal walls of the pores in a thin coating form, orto form nano particles that simply lodge in the pores of the porousstructure.

Some examples of the metal polysulfide (M_(x)S_(y)) materials, solvents,porous structure materials used in the present study are presented inTable 1 below, wherein the following abbreviations are used: chemicallyetched or expanded soft carbon (CSC), chemically etched or expanded hardcarbon (CHC), exfoliated activated carbon (EAC), chemically etched orexpanded carbon black (CCB), chemically etched multi-walled carbonnanotube (C-CNT), nitrogen-doped carbon nanotube (N-CNT), boron-dopedcarbon nanotube (B-CNT), chemically doped carbon nanotube (D-CNT),ion-implanted carbon nanotube (I-CNT), chemically treated carbon fiber(CCF), chemically activated graphite fiber (CGF), chemically activatedcarbonized polymer fiber (CC-PF), chemically treated coke (C-coke),activated meso-phase carbon (A-MC), meso-porous carbon (MC),electro-spun conductive nano fiber (ES-NF), vapor-grown carbon orgraphite nano fiber (VG-CNF or VG-GNF), metal nano wire (M-NW),metal-coated nanowire or nano-fiber (MC-NW), conductive polymer-coatednanowire or nano-fiber (CP-NW or CP-NF).

TABLE 1 Selected examples of the metal polysulfide materials, solvents,and porous conductive structures used in the present study. M_(x)S_(y)Solvent Type of porous structure in the cathode Li₂S₆ DOL/DME CSC, CHC,EAC, CCB, RGO, GO, pristine graphene Li₂S₉ DOL/DME CSC, CHC, EAC, CCB,Nitrogenated graphene, B-doped graphene, Ni foam Li₂S₁₀ DOL/DMEExfoliated graphite, RGO, EAC, amine-functionalized graphene, ES-NF,A-MC Na₂S₂ Tetra ethylene glycol C-CNT, N-CNT, B-CNT, D-CNT, I-CNT,C-coke dimethyl ether (TEGDME) Na₂S₄ TEGDME RGO, B-doped graphene, Nifoam, graphene-coated Cu foam Na₂S₆ TEGDME C-CNT, CCF, CGF, CC-PF K₂S₆TEGDME C-coke, A-MC, MC, ES-NF, VG-CNF, VG-GNF K₂S₄ Diglyme/tetraglymeM-NW, MC-NW, CP-NW, CP-NF K₂S Diglyme/tetraglyme CSC, CHC, EAC, CCB MgS₆Diglyme/tetraglyme M-NW, MC-NW, CP-NW, CP-NF MgS₄ Diglyme/tetraglymeCSC, CHC, EAC, CCB CuS₂ NH₄OH or HCl or H₂SO₄ C-CNT, N-CNT, B-CNT,D-CNT, I-CNT Cu₈S₅ NH₄OH or HCl or H₂SO₄ CSC, CHC, EAC, CCB ZnS H₂SO₄solution CSC, CHC, EAC, CCB Al₂S₃ H₂SO₄ C-CNT, CCF, CGF, CC-PF SnS₂ HNO₃and HCl C-coke, A-MC, MC, ES-NF, VG-CNF, VG-GNF SnS HCl C-coke, A-MC,MC, ES-NF, VG-CNF, VG-GNF

In a Mg—S cell, a proper electrolyte was selected to combine with ananode current collector (Cu foil), an anode layer (e.g. Mg metal foil),a porous separator, a layer of conductive porous structure, and acathode current collector (Al foil) to form a Mg—S cell. The cell wasthen subjected to a first discharge or charge procedure using a currentdensity preferably ranging from 50 mA/g to 5 A/g.

For comparison purposes, several prior art methods were used toincorporate sulfur (the cathode active material) in the cathode layer;e.g. direct mixing of S powder with carbon black particles, physicalvapor deposition of S in a carbon paper (e.g. carbon nano-fiber, CNF),direct mixing lithium polysulfide particles with a conductive filler(e.g. carbon nanotubes), etc.

Comparative Example 18A Chemical Reaction-Induced Deposition of SulfurParticles on Chemically Treated or Un-Treated CNTs

A prior art chemical deposition method is herein utilized to prepareS-CNT composites. The procedure began with adding 0.58 g Na₂S into aflask that had been filled with 25 ml distilled water to form a Na₂Ssolution. Then, 0.72 g elemental S was suspended in the Na₂S solutionand stirred with a magnetic stirrer for about 2 hours at roomtemperature. The color of the solution changed slowly to orange-yellowas the sulfur dissolved. After dissolution of the sulfur, a sodiumpolysulfide (Na₂S_(x)) solution was obtained (x=4-10).

Subsequently, a CNT-sulfur composite was prepared by a chemicaldeposition method in an aqueous solution. First, 180 mg of CNTs wassuspended in 180 ml ultrapure water with a surfactant and then sonicatedat 50° C. for 5 hours to form a stable CNT dispersion. Subsequently, theNa₂S_(x) solution was added to the above-prepared GO dispersions in thepresence of 5 wt % surfactant cetyl trimethyl-ammonium bromide (CTAB),the as-prepared CNT/Na₂S_(x) blended solution was sonicated for another2 hours and then titrated into 100 ml of 2 mol/L HCOOH solution at arate of 30-40 drops/min and stirred for 2 hours. Finally, theprecipitate was filtered and washed with acetone and distilled waterseveral times to eliminate salts and impurities. After filtration, theprecipitate was dried at 50° C. in a drying oven for 48 hours. Thereaction may be represented by the following reaction: S_(x)²⁻+2H⁺→(x-1) S+H₂S.

Comparative Example 18B Redox Chemical Reaction-Induced Deposition ofSulfur Particles in CNTs and Activated Carbon Mats (Chemically Treatedor Un-Treated)

In this chemical reaction-based deposition process, sodium thiosulfate(Na₂S₂O₃) was used as a sulfur source and HCl as a reactant. A CNT-wateror AC-water suspension was prepared and then the two reactants (HCl andNa₂S₂O₃) were poured into this suspension. The reaction was allowed toproceed at 25-75° C. for 1-3 hours, leading to the precipitation of Sparticles deposited in or on CNTs or ACs. The reaction may berepresented by the following reaction: 2HCl+Na₂S₂O₃→2 NaCl+S↓+SO₂↑+H₂O.

Comparative Example 18C Preparation of S/MC and S/CB Nanocomposites ViaSolution Deposition

Meso-porous carbon, chemically treated or untreated, (and, separately,carbon black particles) and S were mixed and dispersed in a solvent(CS₂) to form a suspension. After thorough stirring, the solvent wasevaporated to yield a solid nanocomposite, which was then ground toyield nanocomposite powder. The primary sulfur particles in thesenanocomposite particles have an average diameter of approximately 10-30nm.

Comparative Examples 18 D Preparation of Sulfur-Coated Webs for Cathodes

The step involves deposition of elemental sulfur on meso-porousstructures through, for instance, a sublimation-based physical vapordeposition. Sublimation of solid sulfur occurs at a temperature greaterthan 20° C., but a significant sublimation rate typically does not occuruntil the temperature is above 40° C. In a typical procedure, ameso-porous structure or nano-filament web is sealed in a glass tubewith the solid sulfur positioned at one end of the tube and the web nearanother end at a temperature of 40-75° C. The sulfur vapor exposure timewas typically from several minutes to several hours for a sulfur coatingof several nanometers to several microns in thickness. A sulfur coatingthickness lower than 100 nm is preferred, but more preferred is athickness lower than 20 nm, most preferred lower than 10 nm or even 5nm.

Several series of alkali metal and alkali metal-ion cells were preparedusing the presently prepared cathode. For instance, for the Li—S cells,the first series is a Li metal cell containing a copper foil as an anodecurrent collector and the second series is also a Li metal cell having anano-structured anode of conductive filaments (based on electro-spuncarbon fibers) plus a copper foil current collector. The third series isa Li-ion cell having a nano-structured anode of conductive filaments(based on electro-spun carbon fibers coated with a thin layer of Siusing CVD) plus a copper foil current collector. The fourth series is aLi-ion cell having a graphite-based anode active material as an exampleof the more conventional anode.

Comparative Examples 18E Mixing of Polysulfide with Soft Carbon orCarbon Black Particles Via Ball-Milling

Polysulfide particles and soft carbon (0% to 49% by weight of equivalentS content in the resulting composite) were physically blended and thensubjected to ball milling for 2-24 hours to obtain polysulfide-SCcomposite particles (typically in a ball or potato shape). Forcomparison, untreated or chemically treated SC particles only (withoutpolysulfide) were also ball-milled to obtain ball- or potato-shapedparticles. The particles, containing various S contents, were then madeinto a layer of porous structure intended for use in the cathode.Another series of samples for comparison were made under similarprocessing conditions, but with carbon black particles replacing SCparticles.

Example 19 Some Examples of Electrolytes Used in the Magnesium-SulfurCells

A wide range of lithium salts can be dissolved in a wide array ofsolvents, individually or in a mixture form. In the present study, themost preferred salt/solvent combinations include Mg(AlCl₂EtBu)₂/THF,Mg(ClO₄)₂/THF, EMIC, Mg(ClO₄)₂/PC, and Mg(ClO₄)₂/THD, a recrystallizedMg complex [Mg₂(μ-Cl)₃(THF)₆][HMDS-AlCl₃], (HMDS)₂Mg/diglyme,(HMDS)₂Mg/tetraglyme, and (Mg(BBu₂Ph₂)₂/THF, whereinHMDS=hexamethyldisilazide, PC=propylene carbonate, Bu=butyl, Ph=phenyl,and THF=tetrahydrofuran.

Another well-studied group of electrolytes are solutions oforganomagnesium halides, amidomagnesium halides, and magnesiumorganoborates in a variety of solvents (e.g. THF, or ethereal), andGrignard solutions (RMgX in ethers, R=organic alkyl or aryl group andX=halide like Cl or Br). Useful solutions were produced by reactingAlCl_((3-n))R_(n) Lewis acid with R₂Mg Lewis base in ethers (THF orglymes) at various ratios. All phenyl complex (APC) electrolytesolutions (e.g. in THF), comprising the products of the reaction betweenPhxMgCl_((2-x)) and PhyAlCl_((3-y)) (preferably PhMgCl and AlCl₃), aregood electrolytes.

Magnesium organohaloaluminate electrolytes, produced in situ from thereaction between a Lewis acid and a Lewis base (e.g. a 2:1 mixture ofphenylmagnesium chloride and aluminum trichloride (AlCl₃) in THF) wasalso used. In addition, hexamethyldisilazide magnesium chloride(HMDSMgCl) was also used in the Mg—S cell.

Example 20 Evaluation of Electrochemical Performance of Various Mg—SCells

Charge storage capacities were measured periodically and recorded as afunction of the number of cycles. The specific discharge capacity hereinreferred to is the total charge inserted into the cathode during thedischarge, per unit mass of the composite cathode (counting the weightsof the cathode active material, conductive additive or porous structure,binder, and any optional additive combined). The specific chargecapacity refers to the amount of charges per unit mass of the compositecathode. The specific energy and specific power values presented in thissection are based on the total cell weight. The morphological ormicro-structural changes of selected samples after a desired number ofrepeated charging and recharging cycles were observed using bothtransmission electron microscopy (TEM) and scanning electron microscopy(SEM).

Active material utilization efficiency data from many samples or cellsinvestigated are summarized in Table 2 and Table 3 below:

TABLE 2 Sulfur utilization efficiency data for Mg-Si metal-sulfur cellcathodes containing various S contents, polysulfide coating thicknessesor particle diameters, porous structure materials. Equivalent S %(assuming Cathode Discharge Active 100% conversion from M_(x)S_(y) todischarge capacity, material Sample Cathode active S) and polysulfidethickness or capacity mAh/g (S utilization ID layer material diameter(nm) (mAh/g) weight) efficiency PG-1 Pristine graphene Li₂S₁₀; 90%S; 7.3nm 1376 1529 91.28% PG-2 Pristine graphene Li₂S₁₀; 90% S; 13.3 nm 12931437 85.77% PG-3 Pristine graphene Li₂S₁₀; 75% S; 13.4 nm 1035 138082.39% PG-C-1 Pristine graphene 75% S (PVD) + PG 663 884 52.78% PG-C-2Pristine graphene 75% S; Li₂S₁₀ + PG; ball-milled 694 925 55.24% PG-C-3Carbon black 75% S; Li₂S₁₀ + CB; ball-milled 420 560 33.43% RGO-1 RGOLi₂S₆; 85% S; 13.4 nm 1180 1388 82.88% RGO-1C RGO 85% S, Chem. reaction928 1092 65.18% RGO-2 RGO Na₂S₆; 85% S; 13.4 nm 1120 1318 78.67% RGO-2CRGO Na₂S₆; 85% S; ball-milled 972 1144 68.27% NGO-1 NGO Na₂S₄; 65% S;13.4 nm 887 1365 81.47% NGO-2 NGO K₂S₆; 65% S; 10.2 nm 860 1323 78.99%f-GO-1 f-GO K₂S₈; 70% S; 10.2 nm 989.3 1413 84.38% EG-1 ExfoliatedLi₂S₈; 85%S; 7.6 nm 1286 1513 90.32% graphite worms EG-2 ExfoliatedNa₂S₆; 85% S; 15.4 nm 1172 1379 82.32% graphite worms EG-3 ExfoliatedK₂S₆; 85% S; 14.4 nm 1164 1369 81.76% graphite worms EG-3C CNT Na₂S₆;85% S; 34 nm 940 1106 66.02%

TABLE 3 Active material utilization efficiency data for Mg-S cellcathodes containing various S contents, polysulfide coating thicknessesor particle diameters, porous structure materials. Cathode Equivalent S% (assuming 100% Cathode Discharge Active active conversion fromM_(x)S_(y) to S) and discharge capacity, material Sample layerpolysulfide thickness or diameter capacity mAh/g (S utilization IDmaterial (nm) (mAh/g) weight) efficiency CSC-1 CSC Li₂S₁₀; 90%S; 8.4 nm1348 1498 89.42% CSC-2 CSC Li₂S₁₀; 90% S; 15.3 nm 1287 1430 85.37% CSC-3CSC Li₂S₁₀; 75% S; 16.2 nm 1028 1371 81.83% CSC-c1 CSC 75% S (PVD) + CSC658 877 52.38% CSC-c2 CSC 75% S; Li₂S₁₀ + CSC; ball-milled 712 94956.68% CHC-1 CHC Li₂S₆; 85% S; 13.4 nm 1038 1483 88.53% CHC-c1 CHC 85%S, Chem reaction 940 1343 80.17% EAC-1 EAC Na₂S₆; 85% S; 13.4 nm 10141449 86.48% EAC-c1 EAC Na₂S₆; 85% S; ball-milled 880 1257 75.05% C-CNT1C-CNT Na₂S₄; 65% S; 13.4 nm 999 1427 85.20% C-CNT2 C-CNT K₂S₆; 65% S;11.2 nm 978 1397 83.41% C-CNF C-CNF K₂S₈; 65% S; 14.4 nm 964 1377 82.22%A-MC1 A-MC Li₂S₈; 85%S; 7.6 nm 1208 1421 84.85% A-MC2 A-MC Na₂S₆; 85% S;15.4 nm 1187 1396 83.37% A-MC3 A-MC K₂S₆; 85% S; 26.4 nm 1033 121572.55% M-NW Ag NW Na₂S₆; 85% S; 34 nm 1130 1228 73.33%

The following observations can be made from the data of Table 2 andTable 3:

-   -   1) Thinner M_(x)S_(y) coatings prepared according to the instant        invention lead to higher active material utilization efficiency        in the Mg—S cell, given comparable S proportion.    -   2) For all the cathode porous materials investigated, the        presently invented method of M_(x)S_(y) deposition is        significantly more effective than all conventional methods of        direct deposition of sulfur per se into a porous structure or        direct mixing of either S or M_(x)S_(y) with a conductive filler        to form a cathode structure (e.g. using PVD deposition,        ball-milling, chemical reaction-based deposition, solution-based        deposition, etc.) in terms of imparting S utilization efficiency        to the resulting cathode structure of a M-S cell.    -   3) Chemically treated or activated carbon materials are more        effective than their non-treated counterparts. In addition,        graphene and exfoliated graphite materials are very good        materials for constituting a porous structure to accommodate        M_(x)S_(y).    -   4) Smaller or thinner particles or coating give rise to a higher        cathode active material utilization efficiency.    -   5) The inventive method is capable of depositing a high        M_(x)S_(y) proportion while maintaining a thin M_(x)S_(y)        coating (hence, high active material utilization efficiency).        Prior art methods are not capable of achieving both.

Shown in FIG. 6 are the specific capacities vs. number ofcharge/discharge cycles for three Mg—S cells: one featuring a reducedgraphene oxide (RGO)-based cathode containing solution deposited Li_(z)%coating of the present invention, one containing chemically depositedsulfur (not metal polysulfide) in RGO, one containing a cathodecontaining RGO and Li₂S₈ ball-milled together.

These data indicate that the presently invented Mg—S cell does notexhibit any significant decay (only 5.1%) after 225 cycles. In contrast,the Mg—S cell containing chemically deposited S coating-based cathodesuffers a 28.6% capacity decay after 225 cycles. These results are quiteunexpected considering that the same type of porous graphene structurewas used as the supporting material for the cathode active material andthe same equivalent amount of sulfur was deposited in these two cellcathodes. The cathode containing ball-milled mixture of RGO and Li₂S₈suffers a 59.3% capacity decay after 225 cycles.

The cycling stability of the cathode featuring nano Li₂S₈-preloaded RGOmight be due to the effectiveness of the presently invented depositionmethod to uniformly deposit ultra-thin sulfide coating in the meso-poresin the porous structure. This sulfide was then converted to a uniformthin coating after the battery was fabricated and operation began. Suchan approach appears to also impart an outstanding ability of the porewalls to retain thin sulfur coating, preventing dissolution of sulfurand polysulfide during battery charge/discharge operations.Additionally, as compared to pure S directly loaded in the cathode, theLi₂S₈ coating appears to be more resistant to electrode disintegrationcaused by cathode volume changes. This is likely due to the notion thata M_(x)S_(y)-preloaded cathode layer has already naturally built in someexpanded volume and hence is less prone to sulfur volumeexpansion-induced damage upon repeated charges/discharges

Similarly, FIG. 7 shows the specific capacities (vs. number ofcharge/discharge cycles) for 3 Mg—S cells: one featuring a chemicallytreated soft carbon (C—SC)-based cathode containing solution depositedNa₂S₈ coating of the present invention (pre-loaded Na₂S₈ was convertedto S during first charge/discharge operation of the cell), onecontaining chemically deposited sulfur in C—SC, one containing a cathodecontaining C—SC and Na₂S₈ ball-milled together. The presently inventedM_(x)S_(y) pre-loading approach provides the most cycling-stable Mg—Scell.

FIG. 8 shows Ragone plots (power density vs. energy density) of threecells: a Mg—S cell featuring a MgS₆-preloaded exfoliated graphite (EG)cathode layer, a corresponding Li—S cell, and a Mg—S cell featuring acathode layer containing solution-deposited sulfur. These diagramsindicate that the presently invented metal polysulfide-preloaded cathodeapproach is significantly more effective in providing the Mg—S cell witha high power density and high energy density as compared to theconventional approach that directly loads S in the cathode. The Mg—Scell exhibits significantly higher energy density and power density thanthe Li—S cell made according to the same polysulfide-preloadingapproach.

In summary, the present invention provides an innovative, versatile, andsurprisingly effective platform materials technology that enables thedesign and manufacture of superior magnesium-sulfur rechargeablebatteries. The Mg—S cell featuring a cathode containing a conductive,meso-porous structure with ultra-thin M_(x)S_(y) pre-loaded thereonexhibits several unexpected characteristics: a high cathode activematerial utilization rate, high specific capacity, high energy density,high power density, little or no shuttling effect, and long cycle life.

1. A rechargeable magnesium-sulfur cell comprising an anode activematerial layer, an optional anode current collector, a porous separatorand/or an electrolyte, a metal polysulfide-preloaded active cathodelayer, and an optional cathode current collector, wherein said metalpolysulfide-preloaded active cathode layer comprises: (A) an integralporous structure of an electronically conductive material, wherein saidintegral porous structure has massive surfaces having a specific surfacearea greater than 100 m²/g or has pores with a size from 1.0 nm to 100nm and wherein multiple particles, platelets or filaments of saidconductive material form a 3-D network of electron-conducting paths; and(B) a sulfur-rich metal polysulfide, M_(x)S_(y), preloaded in said poresor deposited on said massive surfaces, wherein x is an integer from 1 to3 and y is an integer from 1 to 10, and M is a metal element selectedfrom an alkali metal, an alkaline metal, a transition metal, a metalfrom groups 13 to 17 of the periodic table, or a combination thereof;wherein said metal polysulfide is in a form of solid-state thin coatingor small particles with a thickness or diameter less than 50 nm andoccupies a weight fraction of from 1% to 99% of the total weight of saidporous structure and said metal polysulfide combined.
 2. Therechargeable magnesium-sulfur cell of claim 1, wherein said metalelement M is selected from Li, Na, K, Mg, Ca, Zn, Cu, Ti, Ni, Co, Fe,Mn, Mo, Nb, Ta, Zr, or Al.
 3. The rechargeable magnesium-sulfur cell ofclaim 1, wherein said M_(x)S_(y) is selected from Li₂S, Li₂S₂, Li₂S₃,Li₂S₄, Li₂S₅, Li₂S₆, Li₂S₇, Li₂S₈, Li₂S₉, Li₂S₁₀, Na₂S, Na₂S₂, Na₂S₃,Na₂S₄, Na₂S₅, Na₂S₆, Na₂S₇, Na₂S₈, Na₂S₉, Na₂S₁₀, K₂S, K₂S₂, K₂S₃, K₂S₄,K₂S₅, K₂S₆, K₂S₇, K₂S₈, K₂S₉, K₂S₁₀, MgS, MgS₂, MgS₃, MgS₄, MgS₅, MgS₆,Mo₆S₈, Nb₆S₈, Zr₆S₈, or Ta₆S₈.
 4. The rechargeable magnesium-sulfur cellof claim 1, wherein said M_(x)S_(y) is loaded in said pores or on saidmassive surfaces after said integral porous structure is made and beforesaid cell is made.
 5. The rechargeable magnesium-sulfur cell of claim 1,wherein said integral porous structure is a meso-porous structure formedof multiple particles, platelets, or filaments of a carbon, graphite,metal, or conductive polymer, wherein said meso-porous structure hasmeso-scaled pores of 2-50 nm and a specific surface area greater than100 m²/g and wherein said carbon, graphite, metal, or conductive polymeris selected from chemically etched or expanded soft carbon, chemicallyetched or expanded hard carbon, exfoliated activated carbon, chemicallyetched or expanded carbon black, chemically etched multi-walled carbonnanotube, nitrogen-doped carbon nanotube, boron-doped carbon nanotube,chemically doped carbon nanotube, ion-implanted carbon nanotube,chemically treated multi-walled carbon nanotube with an inter-planarseparation no less than 0.4 nm, chemically expanded carbon nano-fiber,chemically activated carbon nano-tube, chemically treated carbon fiber,chemically activated graphite fiber, chemically activated carbonizedpolymer fiber, chemically treated coke, activated meso-phase carbon,meso-porous carbon, electro-spun conductive nano fiber, highly separatedvapor-grown carbon or graphite nano fiber, highly separated carbonnano-tube, carbon nanowire, metal nano wire, metal-coated nanowire ornano-fiber, conductive polymer-coated nanowire or nano-fiber, or acombination thereof, and wherein said particles or fibrils areoptionally bonded to form said porous structure by a binder of from 0%to 30% by weight of a total porous structure weight not counting themetal polysulfide weight.
 6. The rechargeable magnesium-sulfur cell ofclaim 1, wherein said integral porous structure is a porous graphenestructure containing a graphene material or an exfoliated graphitematerial wherein the graphene material is selected from pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, boron-doped graphene, nitrogen-dopedgraphene, chemically functionalized graphene, or a combination thereofand wherein the exfoliated graphite material is selected from exfoliatedgraphite worms, expanded graphite flakes, or recompressed graphite wormsor flakes, and wherein said graphene structure comprises multiple sheetsof said graphene material or multiple flakes of said exfoliated graphitematerial that are intersected or interconnected to form said integrallayer with or without a binder to bond said multiple sheets or flakestogether, wherein said binder is from a resin, a conductive polymer,coal tar pitch, petroleum pitch, meso-phase pitch, coke, or a derivativethereof and occupies from 0% to 30% by weight of a total porous graphenestructure weight not counting the metal polysulfide weight.
 7. Therechargeable magnesium-sulfur cell of claim 1, wherein said integralporous structure is a porous, electrically conductive material selectedfrom metal foam, carbon-coated metal foam, graphene-coated metal foam,metal web or screen, carbon-coated metal web or screen, graphene-coatedmetal web or screen, perforated metal sheet, carbon-coated porous metalsheet, graphene-coated porous metal sheet, metal fiber mat,carbon-coated metal-fiber mat, graphene-coated metal-fiber mat, metalnanowire mat, carbon-coated metal nanowire mat, graphene-coated metalnano-wire mat, surface-passivated porous metal, porous conductivepolymer film, conductive polymer nano-fiber mat or paper, conductivepolymer foam, carbon foam, graphitic foam, carbon aerogel, carbon xeroxgel, or a combination thereof.
 8. The rechargeable magnesium-sulfur cellof claim 1, which is a free-standing layer or is physically orchemically bonded to a current collector layer prior to beingincorporated into said magnesium-sulfur cell.
 9. The rechargeablemagnesium-sulfur cell of claim 1, further comprising an element Z or ametal compound M_(x)Z_(y) deposited in said porous or on said massivesurfaces wherein said element Z or M_(x)Z_(y) is mixed with said metalpolysulfide or formed as discrete coating or particles having adimension less than 100 nm and said Z element is selected from Sn, Sb,Bi, Se, and/or Te, and wherein x is an integer from 1 to 3, y is aninteger from 1 to 10, and M is a metal element selected from an alkalimetal, an alkaline metal, a transition metal, a metal from groups 13 to17 of the periodic table, or a combination thereof, and the weight ratioof Z/M_(x)S_(y) or M_(x)Z_(y)/M_(x)S_(y) is less than
 1. 10. Therechargeable magnesium-sulfur cell of claim 1, wherein said metalpolysulfide occupies a weight fraction of at least 70% of the totalweight of said porous structure and said metal polysulfide combined. 11.The rechargeable magnesium-sulfur cell of claim 1, wherein said metalpolysulfide occupies a weight fraction of at least 90% of the totalweight of said porous structure and said metal polysulfide combined. 12.The rechargeable magnesium-sulfur cell of claim 1, wherein said metalpolysulfide thickness or diameter is smaller than 20 nm.
 13. Therechargeable magnesium-sulfur cell of claim 1, wherein said metalpolysulfide thickness or diameter is smaller than 10 nm.
 14. Therechargeable magnesium-sulfur cell of claim 1, wherein said integralporous structure has massive surfaces having a specific surface areagreater than 500 m²/g.
 15. The rechargeable magnesium-sulfur cell ofclaim 1, wherein said integral porous structure has massive surfaceshaving a specific surface area greater than 700 m²/g
 16. Therechargeable magnesium-sulfur cell of claim 1, wherein said electrolyteis selected from polymer electrolyte, polymer gel electrolyte, compositeelectrolyte, ionic liquid electrolyte, non-aqueous liquid electrolyte,soft matter phase electrolyte, solid-state electrolyte, or a combinationthereof.
 17. The rechargeable magnesium-sulfur cell of claim 1, whereinsaid electrolyte contains a salt or salt/solvent solution selected from:(a) Mg(AlCl₂EtBu)₂/THF, Mg(ClO₄)₂/THF, Mg(ClO₄)₂/PC, Mg(ClO₄)₂/THF, arecrystallized Mg complex [Mg₂(μ-Cl)₃(THF)₆][HMDS-AlCl₃],(HMDS)₂Mg/diglyme, (HMDS)₂Mg/tetraglyme, or (Mg(BBu₂Ph₂)₂/THF, whereinHMDS=hexamethyldisilazide, PC=propylene carbonate, Bu=butyl, Ph=phenyl,and THF=tetrahydrofuran; (b) a solution of an organomagnesium halide,amidomagnesium halide, or a magnesium organoborate; (c) a Grignardsolution, RMgX in an ether, wherein R=organic alkyl or aryl group andX=a halide element; (d) a reaction product of a AlCl_((3-n))R_(n) Lewisacid with a R₂Mg Lewis base in an ether; (e) a solution comprising aproduct of the reaction between PhxMgCl_((2-x)) and PhyAlCl_((3-y)); or(f) a mixture of phenylmagnesium chloride and aluminum trichloride,AlCl₃, in THF.
 18. The rechargeable magnesium-sulfur cell of claim 1,wherein said electrolyte contains a salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂, Lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate(LiBF₂C₂O₄), Lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates(LiPF3(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), anionic liquid salt, sodium perchlorate (NaClO₄), potassium perchlorate(KClO₄), sodium hexafluorophosphate (NaPF₆), potassiumhexafluorophosphate (KPF₆), sodium borofluoride (NaBF₄), potassiumborofluoride (KBF₄), sodium hexafluoroarsenide, potassiumhexafluoroarsenide, sodium trifluoro-metasulfonate (NaCF₃SO₃), potassiumtrifluoro-metasulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI),bis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂), or acombination thereof.
 19. The rechargeable magnesium-sulfur cell of claim1, wherein said electrolyte contains a solvent selected from ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xyleneor methyl acetate (MA), fluoroethylene carbonate (FEC), vinylenecarbonate (VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),Poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, roomtemperature ionic liquid, or a combination thereof.
 20. The rechargeablemagnesium-sulfur cell of claim 1, wherein said anode active materiallayer contains an anode active material selected from pure Mg metalhaving a Mg content no less than 99.9%, or a Mg alloy or compound havinga Mg content from 70% by weight to 99.9% by weight.
 21. The rechargeablemagnesium-sulfur cell of claim 1 wherein said cathode has an activematerial utilization efficiency no less than 80%.
 22. The rechargeablemagnesium-sulfur cell of claim 1 wherein said cathode has an activematerial utilization efficiency no less than 90%.